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The Hitchhiker's Guide to Ruby GC

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The Hitchhiker's Guide to Ruby GC
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When Ruby programs slow down, the usual culprits—database queries, superlinear time complexity—aren't always the real problem. Ruby's object space and garbage collection are a surprisingly rich and oft-misunderstood area of the language, and one where performance issues can easily hide. This talk is a brief but deep dive into the history and details of garbage collection in Ruby, including its evolution, parameter tuning, and a case study using the Unicorn web server.
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what you hear it. rock a fool would get started. so yes it's acres guides you reach the sea. i like justin speak very quickly so if i do speak i get going to get excited because your risk really exciting and garbage such as cutting if i start talking way too fast because he just kind of like a no like some kind of something about that exactly please. that the awesome so unlike just and over i don't have any tax insights for you. i do make silly jokes like and does but unlike air and i knock it off the life changing insights into the virtual machine to my talk is in fact up literally garbage or about coverage. speaking of did you guys here but is this the controversy about the showed dirty jobs. maybe it just me basically i guess it's going to do with the huge number of micro services that they are employing. you can yet thank you so many lap that i could get the i also don't enjoy my pounds nearly like as much as areas so i'm going to win just put a line through that i'm not going to do that anymore. now anyway very cool so yes i should have to say i'm learning a lot was a terrible job i'm learning a lot of this conference and learn a lot that really count. and i'm trying to apply immediately so you know gary had a great talk about ideology and belief in its one of the things that i'm sort of trying to internalize but you know it's hard right like we don't know we don't know and even we know it sometimes we are unable to disarm marcel so. this is my first time getting this talk is actually my second talk ever i spoke earlier this year that i really got some going to try to confront my own imposter them and also channel air and and not die so and also you'll see that my nervousness manifests in of noxious slide transition so. there's that the way so is aired weinstein. i work at a company called cond nast out of you guys have heard of counting you're probably familiar with various brand so there's no wired the new yorker vogue g.q. etc i'm i really like reading really great jobs get a lot of work so it's nice to get the right ready for some of those projects and and on my own time as well. the contest entertainment folks actually are. all using reveal so we are hiring you'll get zip wire and you might be interested sort of but as strange passion and and wise to hash and not an object you know i got a person got new and and that is foreshadowing so you'll see there's a lot of literary devices. in this talk.
so also obligatory self promotion of the curriculum and could get me a couple years ago and i also wrote this book. called really was agree it's for kids ages eight to twelve and there's a really great birds of a feather organized by the government to actually doing talking with a look up later in this very room so highly encourage you to go that talk as well as interested in learning to be teaching the kids really nouns please come. see me after the show and i am so no starches has been delightful is offering a forty percent off if anyone does what the book recounts twenty fifteen will be forty percent off i think for the week. garbage collection so there's a lot of mythology around d.c. and easy to doing in sort of total works it's actually not as bad as it seems to those not familiar the don't panic as kind of like emblazoned in large friendly letters on the cover of the attackers got to the galaxy this is also as much.
my benefit is yours. so i pods year because you know this is a computer to companies and we should start as you're really is not slow. yes sort of sometimes depending it can be but not for the reasons that you think some people say well you know the program has slowed must be a day's thing or it must be there's some super when you're crazy like forgiveness the loop but i'm doing something but as in there that i should be doing or reason interpreted language you can't possibly be fast and we should all be using go.
your job or something. and so really what i found is that under the program slowdown these aren't always or even often the culprits. the public space and and she's actually extremely rich part of the language and not surprisingly when you have a lot of richness there's a lot of nuance and performance bugs can hide their. these things are true. but on screen you know that this sort of you make some operations or than no job or compiled language but the reason that we have all this and.
better and worse. because everything is an object that not everything you have locks not objects but you get the idea. so it's taken time and talk about the history of garbage collection in derby. first we're talking about m.r.i. see really we're not talking about revenue surgery which have different garbage collectors had wanted to include in this talk but unfortunately i only have so much time and ended up on the cutting room floor i also don't know nearly as much about them but if you are interested in reading his surgery be or have some cool stuff to share please a company and.
i mean a comparison city's alternative limitations if the comparisons make sense that my kind of make disappearances but by and large the talk is about them right. so it's not what really went seven.
twenty seven uses tracing was a reference getting g c n n g c traces are sort of like you're looking for reachable objects in the object raf by traversing the object and i can get to one that object is reachable and great that should not be collected they're still references to it as. has demonstrated the raf and if they're not reachable have not been reachable it is eligible for collection. one in seven use very simple mark and sweet market was invented by. john john mccarthy that's right it's not as an agreement. nineteen fifty either fifty nine and its it's astounding that rugby has had gotten so i'd gone so far which is simple mark and sweep garbage collection and got a question works this way and in markets we really will allocate some about memory will talk about details and little bit when there's no more free memory.
this is ok great i go through i'm going to look for all of the act of objects that i can find a mark them as active and then anything that's an active can sweep onto a new list and that's world go when i need more stuff. that's a broad the this is what he's doing it that this was a fun animation i like the air is going anywhere so yeah the important thing to realize though when you're in your marking in sweeping and markings we think having a great time is that everything stops. because rich ability to and the object craft can change from you know when you are marking and then sweeping it can change while borders executing industrial the program is expanding. so in order to markets with the collector has to stop the world if you hear people talking about stop the world garbage collection major garbage collection this is what they're talking about everything stops be marks sweeps it is great and continues execution.
so in winter we got some improvements we got lazy markets we and this is an improvement because it increases or rather reduces the pas time by saying ok i'm actually is going to sweep in phases. it doesn't really do anything to the overall amount of time you spend quite a garbage but if you say i'm going to garbage collectors going to happen second and to ten times that would once again doing five times you know you don't get these i'm an unacceptable and times while reflection happy and if you do have a highly of him to drive your application you have a web server you have something the ugly and you can't. it's like to sit there for a couple seconds were collecting garbage this is a big win and unfortunately when he seven and one and three both subvert need of copyright which will also talk about it will be on the quiz that's a huge letters and was just more information. it was a dozen reduce the kind of overall pain of stopping the world that exists or amortize. more more sweeps.
so in into point so we've got but marking so we're no longer marking objects directly rather we have had a bit map that represents the state of objects and eligibility for collection. this will be extremely important leader because this is what sort of allows us to to use copyright and have a great time to be covering all these in-depth pretty soon i was kind of want to show you were going on the journey.
that is this talk to so yes so if like air and i don't die during his presentation be turning thirty in march which is kind of like when i become old i guess i guess each ticket is a collection on titan broadly about ready to one so we have generational garbage collection two generations a young one an old one. and if you survive to collections into one you become old and this is sort of based on the was called the week generational i thought this is objective die young and you have a lot of objects that that appear and do something and in oregon and so based on the fact that objects to die young it makes sense to you kind of fast minor g.c. frequently and the slow or stop the. old collections less frequently. and if you're just in the argentines younger them kg has a talk from europe coheres ago which is excellent and there's a link at the end of this presentation.
and so that was sort of the history of the twenty seven one nine and then two days to one and now we're going to talk a little bit more in-depth about two to two but it's two two in general and will talk a little bit more about copyright that networking all the stuff i mentioned but here is going to focus quickly on two things. incremental and symbol of the sea which we haven't you will also talk a little bit about garbage collection tuning which is scary and amazing i'm so simple g c i knew i could read she was an excellent excellent right up on simple g c n n if you've read that have you been.
writing large rolls up the rules are really applications are familiar with this and this notion of like so simple denial of service right you have a bunch of symbols they never get elected because they just don't their life forever if you are to do something strange but not on the face of it silly which is to approach the let users generate something that will create a symbol and someone create south. and thousands and thousands of similar you will eventually run out of every that is bad it turns out and so we have now is the ability to reclaim some symbols not all them and others have collected when the americans with no longer need the reason that i say some not all is because really internally will generate symbols for safe method names and you could. easily das yourself again if you did something like this dynamically generate a lot of methods that will get their own symbol that unusual anyway so we have that which is nice but the really cool things we have incremental major d.c. get over them that others excellent blog posts of papers on encourage you to look up. the cool thing here is the sort of tricolour marking and many of your probably familiar those ideas that we have three types of objects we have white which are unmarked grave which are marked and they refer to somewhat objects and black which are both market but our market do not refer to any white objects. and here's how the over them brother works you say all the objects are white and everything it's alive obviously life like things on the deck. those are great and i'm going to start picking up going to pick one great objects and regulators roll them to visit everything reference and color that great and when i'm done with this and go back to that first great objects ok this is no black since it does not refer to any white objects going to keep doing this and you doing this until i only have black and white objects in this tells the ok great i have like. objects which are obviously a line and white which are obviously available for collection so many go through and sweep. and that's sort of how the other than works is their part the part where we go through and we do all the colour changing and we change the original no back to black this is sort of what's going on in the in the incremental part of the over them like revealed his execution and it will do some sweeping and execution and and so on. but there's a bug.
and i were in this room air modi so yes i wanted a lot of this comes. we're going to get to dance around that was about it anyway so what's the bug the problem is what if you have a white object appear so we create new object and there's no great object with references to it because they're only black and white objects.
we are going to inadvertently reclaim this logic that stick it will be white when the river runs the next time you finish his excuse me because it's so this is a white object is available for me to collect and our life object that we wanted is gone. this is a bad thing so what do you do he said so there's this whole thing called right barriers there should be effective you know it because it says on the screen.
basically it for a lot of complicated reasons there are insufficient right barriers in in see ready and that is the thing that you should ask reach about or somebody who knows much more at verizon i do not unfortunately i write barrier scientist but basically it's sort of interest we don't have great very protected and write very are unprotected. the objects what we love playing here is ok because the pos times relative to the number of living right very are interested objects and most objects are. your legs define one strings raise hash of things like that are right they are protected possum is not going to be that that so what is the actual fix her with her like ok this is not going to be the tricky we can apply this to fix a problem essentially after all the black and white objects are cool are identified right before we collect the white. once we want to go and say ok i'm going to scan from all the unprotected black objects we can actually guarantee that the ones that are protected are managed and just kind of do another check like has anything basically change have new white objects appeared since i last checked and by doing this we kind of soca great now i'm not going to have this bug right inadvertently collect the objects said. can run going to have a problem with losing things that i really didn't want to love to cook.
so that sort of the a brief history of the vichy intervene from what seven to two to the we cannot talk about d.c. tuning and if there's sort of if there's one teal the arc is one thing you take away from this talk it should be the slide. do not do it or other directors only don't do it yet and this is the paraphrasing about michael jackson that not the michael jackson michael a jackson was also a michael jackson. but basically said yes. in terms of program a position any kind of opposition don't do it and if you really what we're doing ok fine but don't know yet. so what happens if we said we're just going to try those so here if you the variables that you can modify and in rugby to sort of effect howell garbage collection is performed these first to the heap growth factor and keep growth next lots lowering either of these will trigger more frequent young object garbage collection.
there's actually saying hey like when i get new hips which will talk about a second. you can i think the default in them when nine or two may still be one point eight but at one time was one point eight so you know you have ten thousand the number ten thousand and then ok it's going to one point me to get eighteen thousand and this will say it as you need more memory great here's here's larger and larger chunks of memories so you can lower this and then you run out of memory faster and your forced to collect. the same thing with the that growth next lots of the latter three malik limit the malik limit max and the limit growth factor lowering any of these three will tell her be that it's not allowed to allocate as much off the memory before running my energy see which also triggers more frequent object allegations or collections rather and you can sort is with old malik which is i think innings. as for larger like major jeezy but this is probably about idea because as we've seen if you started a lot of major garbage collection you're going to be sitting there like a garbage instead of that you send an e-mail to users or. something of that nature.
the important to realize this is not a silver bullet there are no silver bullets there are a lot of where once there are no silver bullets its it's about time so there is. you know this is a also not as helpful as it may seem there were times we say yes there's a lot of objects as a lot of stuff is going were turning and garbage collection is a problem with identify this is kind of thing that we should do you or you can inadvertently make things worse for yourself modify these variables and not doing a lot of measuring being very careful read this is also one of those myths is very attractive it's the source of those that were told. tricks and tips about x.y.z. there's a lot of mythology around things like that to doing and and garbage collection and advised that if you do decide s. and into the garbage collector this is something you just want to measure lot beforehand after make sure that what your measuring is actually we're measuring has often times it is not the case and just kind of proceed with caution. so that said we're going to move on so you think the sort of the book which is this this case study artery.
so couple years ago i was working a company and we had over the application was interesting is of art was it was not a real application was a suite of seven sinatra applications had been sort of. smashed together interesting ways. and with his work is no users would rise to the site they would go to the web blair and the the republication would make requests to a number of jobs services jobs there is also based on social service would say hey i have some reason for you and your sales and that's great of days on him and would have been fleeting dashes and the past summer. and so you just have these huge cash is floating around and you know people would just start picking stuff out of them or mutating them because that's the thing you can do. and it was a bad time and so someone got this is really crazy idea and said why don't we take some objects and orient our code around them. i suspect this is a fat but we want our tribe because this actually is not working out now you see why earlier there's ash not a person that got news up because hatches are think so anyway.
we were talking about doing this and we started doing it and performance tent we were constantly running out of memory were sluggish we're having all sorts of issues and relic every was having a hard time the business will get us reeling at each other and we couldn't figure out why we decided to write object or in a derby we shot ourselves in the foot. so let's talk about memory and again this is all in the context of one in three of the time you're running to be one and three and you're sort of trying to figure out what's going on in this is sort of how to switch to to happen.
so the memory model looks something like this really objects are forty bite our values structures which reallocates and heaps of sixteen killed by speech. this is for sixty four bit architecture i suppose for thirty two bit going to be somewhere like twenty or twenty four but will save for argument's sake that we're all using sixty four bit machines so you can get somewhere on the order of four hundred objects people because you're sixteen thousand whites divided among forty by its part of the structure and so you really will give you about one hundred fifty heaps to start with. johnson.
so you can kind of test yourself here was inside the senator at you can see the objects space there's somewhere it is about four hundred eight objects pretty so.
in that four hundred ballpark and it turned out were making a huge there's little harder he apologized a huge number of objects so obviously the x. our consulates just miles for been lives at great the council task starts i are be with the application loaded we say gee start great and the stat and the interesting things in here are we see the number of total jeezy runs. we see the number keeps with at least one used slot the place is the total number of peeps and you can command is how many more troops to ask for which i think in this yet in this version of reviews was at one point eight times the previous number but the most interesting thing is this the heap live number they were half a million objects just too. start were even doing anything we just started a web service and the audience either this happened i would say that for comparison there's an average rules that would be somewhere closer to four hundred thousand but that sort of nonsense that is that right but there's no average rose happy over different business needs of different versions of rails for a different versions of really are different really implementations different web servers of whom are unicorn. their or or thin this was the seemed completely wrong for his son a trap for this like smashed together frankenstein's monster senate trap. and we were even using active record we were just talking to the services and and getting responses. so we let's talk a little about these are values for small values resource them directly on the object and you might have heard shots he has an excellent book or be under the microscope. that there's in this is very interesting against those weird old tips rights like you don't want strings that are over twenty three characters this is crazy you it doesn't matter if you actually really do care about that level of performance in opposition to his company after the talk because i have a tremendous amount to argue that the awesome but essentially. what happens as yet twenty two characters your view of the value directly twenty four larger you have a pointer to some or other location memory and this is a much lower the sober for large values for you whether it's a stringer a rational or values actually point or so your value itself has a flags field which can to. and f.l. mark which will talk about the contents which is the value or pot it's either actual object or it's a pointer to one or you have next which is a pointer to the next an archer. and there's are stricter strings are actually hatches are object for custom objects and users based on these are all the same size. so we talk about heaps so in one in three we were seeing we had ten thousand slots which one he got second heat and which is another ten thousand slots and every time we needed to keep we are multiplied by one point eight so the third time reduces pay any more memory said it in great we get another eighteen thousand so now we have thirty thousand after the initial twenty thousand and then if we have to ask again. we go from thirty thousand two hundred six thousand four hundred because we're doing this kind of doubling almost as one point eight that factor. and it can be tricky it and this isn't a case for tuning is one thing that we did do because you know what if you say and i really like fifty thousand objects and so i asked her work a dozen spots otherness for more than one hundred six thousand four hundred and i'm never actually going to use these and you realize you know really is not going to get back to the every system into the process as it's so not asking for more memory that. you need can be tricky and is that there's a case to be made her for doing the kind of tuning i warned you about whatever. so let's talk more about markets we are really keeps comprised lists of our values list were in fact invented by alexander graham link eighteen thirty six i realize that if you didn't see as doctors to sounds like crazy stuff so i'm sorry about that. so when there are no more free our values re one and will settle mark on all activity objects this is the marking phase and then reeling small the act objects kind of sweets them into a single link list in this is called the free list. theirs is the frequency and imagine it in your ear minute a link list marked objects summer not work and ones are not marked are available to be reclaimed so he gets what. and used again.
i just love that nation. so copyright we talk about this little bit ago copyright the idea is to win your production process or a process called for the new child process years on memory with a parent and incomes are only made when the right is forced which is cool this is great you don't actually have to write anything unless there's something different and so the for process and apparent as long. you're not mutating stuff they can share memory and its you have the sort of persistent data structure is really cool. the problem is if you mark an object directly and say you are marked you are available and then there's a knowing that at the trial process somewhere that the object is you know is not marked so you have martin not marking of these objects were proliferate that only differ in their eligibility for collection and this is that you kind of get a proliferation of. objects this kind of subversion of the copyright and its it's not necessarily true in all places on linux box and production on you know your limits machinery or a sex machine they do leverage copyright will assume that it's true everywhere there are some his words not but basically now we have all these objects floating around and for a web server. like unicorn that does concurrency via forking the more unicorns you have the worship problem gets because you're constantly writing new objects. so this is the air that portion of the presentation you can tell your friends and colleagues and co-workers that you learn something from my talk just a quick picture of those from the tempest idea being ok we don't need to do these rights we don't need to have all this so what's not. and so this leads to memory and it networking is measured before they would be to go so every feet now has a header that points to have it now and you can kind of it's a little hard to see but these ones and zeros correspond to mark or not markets in the actual heaps lot so you no longer have to mark an object itself as marked you can just updated that map and that been at the track. who is available for collection of those not. so this header is no one is what is martin and there's an art. and so ready to go with his head or just kind of simple thousand was actually leverage copyright that you see mark no longer modifies objects and we only have one object it doesn't change in terms of its all those reflection we don't have to write it among a luxury for processes which have one. its mission because unicorn manages everything by forking the more you of course you have the worst the problem gets now is not a bad thing you know unicorn is a popular review observer forking is not intrinsically bad you corn is intrinsically that reason that interest me that really one entries garbage collection of them was universally doing a bad thing on which. she's been fixed script was mentioned as an increase is the problem gets worse so here's numbers hopefully their somewhat eliminating a loading application and one in three invoked hundred twenty two g.'s he runs and took about four point four seconds and then changing nothing else will only happen to vote sixty six she runs and took about three seconds. the one in three and we were looking at the stats and have been training and and carefully checking and there was some variation terms of genders and stuff like that we try to get all that juggle out it was about forty seven percent more time simply collecting garbage in his application. so again this information. what we do as much pressure expect the number one is we have created to release it so we could leverage copyright for that requires also faster as hard to see that the line there this was new at the time it's not so now but there are procedures like model pretend lazy raiders refinements and to talk or finance which is awesome unfortunately you're missing it but don't be a record. doing so that's good. it was about time like the one in three regions of life in february and you know we were talking about this i think the previous december we have really a couple months to get to really get wrapped up and yet so that was the person that we did was make sure we were under the two so another takeaways up a grubby so this christmas are getting ready to three and will be awesome to. number two is we profiled so i imagine we try to find eliminate sources of load was been a lot of time doing if you see profiling this is the read to docs there's also a doctor to want to thank urge you to take a look at the sea and sort of the methods that are available there there's a lot of information about what really is doing much like how air and kind of showed in his talk like how you can kind of. poll part see all the instructions in that the yard is executing you can also go through and see what object of being allocated how long their lives only slight you have things that asians really very cool. we also use the really program which helped us and i can say i don't have enough good things to say about it it's was awesome and thirteen yes i know i said don't do it but we reasonably sure we knew we were doing it turned out well so far so good basically between the d.c. so the three variables that we touch there was the malik limit which controls when you fall for. jeezy run have always eight megabytes which i think was set in something was chosen by god to ninety five which made town isn't sense for ninety ninety five the didn't make sense for us so we can never want to get more memory and sore point on full stop the world you see because we could you do more stuff before we had to actually that the number to the he means lots. the controls the slots party which cost ten thousand we tune did a little bit to get more objects perky i want to say we only modified to about it was twelve thousand something of that nature and then be three start of a three thirty plots growth factor as much in the growth factor was one point eight so we would ask for more and more and more we can look at the graph and realize that we were kind of plateau. a little bit and we didn't really need to do one point eight we think we select one point one point two million lowering that interview to will give you more frequent. and of the she runs for the for the young generation for sure you know the minor matter to see you have a full set the world stuff. so some credits and some further reading i'm deeply deeply indebted to all these people to catch on as he has said his excellent book. we're going to a microscope was very helpfully have plenty of parting with happening since after a great post at the idea is to find urgency to a couple years old but still a great read passbook dimas a book called that reforms opposition which goes much more debt than that stuff i kind of talk about the beginning. where lot of the issues that we see in a review of occasions have to do with the riches of the attic space and garbage collection and sort of what is around and his is less related necessarily to go from bad algorithms or bad dates craze of those things do exist and then again the stock is just astounding i encourage you to to look that up as well what goes on the. a hurricane your blog videos on you tube there are all excellent and so really think so much for your time i really appreciate your taking time from your of the cop to come within me talk about stuff and in public a tory shameless self promotion this is me is so i work for the that's the book that i wrote.
and thanks so much i. ak. i'm sorry i don't understand a question about a band you see that's a great question so that was saying that we were trying to figure out for this application as it pushes out of n.g.c. what i think about it we the idea is you century tell the garbage collector you're not in charge of when you run i will tell you when you're allowed to run this is the thing that is useful if you have something like a web application where their requests. coming in and someone could be in the middle of very complicated request and reveals a timeout amount of memory any to collect all your back and adjust to a major average collection rate middle you don't happen as it is you say i'm going to only allow garbage collection in between in between request that maybe every ten requests are under question regret request that's what i would like to do some garbage collection. and then back to back to show a for unicorn at the time the tools available where unicorn worker killer and i think there is another out of n.g.c. a library that we're using that sort of like allowed us to maintain memory while not want to kind of sniping like big load old unicorns that would that happened since you were telling d.c. not to have any more like. i will tell you when your lives that something was a lot of overhead doing that never imagined like you century give up all the nice cool like automatic never imagined that to give you and it's a lot to fit in your head so why i think it's there are certainly times where it makes a ton of sense and can be very useful we found it was too hard for us to get right which is not mean it's not too hard for you in your computer. how long should have major garbage collection take that is an excellent question. i feel ok i look i don't know the answer so major of the questioning certainly don't want to be like spending several seconds collecting average as as gay marriage is sophistry like this was sort of like why he was not convinced that great car because she was necessary because jobs are hard question in two thousand to was taking several seconds and it was unacceptable. with the newer stuff into two and the court ability to get incrementally do it. i feel like there's. the applications ever done it was like less than a second look more than a second we're kind of goes is taking a long time ago it's what's actually going on but it's good it's going to depend on your application in your needs. where do you know the book yes like as that of the flashing by now been so you can get the book from your local bookseller it's a published by no start press to build on amazon is available of arsenal i wish i had copies with me but they were did they are extremely heavy legged something like three hundred and forty pages like it's surprisingly big. so yeah it's pretty cool i am i be happy to show anybody like a p.d.f. for you know some simple pages on my machine later say look i wish i had a copy that i'm not good at marketing firms correct dead so this is that yes the to be clear that's exactly right to question so the question is is this code is for no starch and that's correct so this is not saying that you can use of any bookseller. if you go to know starts dot com and use an autobiography was agree there's a little boxes do a promo code if you put into the conflict when he fifteen it will be forty percent off like i said it's for kids ages eight to twelve certainly young six or seven if they've got left the really motivated and her parents are going to help because it can be kind of tricky and i suppose up to high school but you know what sort of like a phantom tollbooth. like there's a time to read it at that time is not maybe when you're sixteen but i had adults tell me that they liked it so if you want to energy maybe itself. aka million thanks so much. ak. the.