+Ulan Degenbaev <u...@chromium.org> On Wed, Apr 11, 2018 at 10:57 AM <hpa...@chromium.org> wrote:
> Thanks John for your detailed report. > > You are right, incremental marking steps should ideally be spread out over > the whole program execution. It seems like the incremental marking task is > the only task running in the observed trace. Do you have any JavaScript > work pending? JavaScript should take turns with incremental marking work. > Is your workload somehow reproducible for us? > > We have some good news for you: V8 6.4 introduces concurrent marking which > moves most of the marking work on a concurrent thread. I would be > interested in seeing how your workload performance changes with a more > recent V8 version. > > > On Wednesday, April 11, 2018 at 10:17:01 AM UTC+2, John Edwards wrote: >> >> Hi all! Just want to lead with a quick thanks to everyone who makes v8 >> happen! It's been a super useful piece of technology for me for many years. >> That said, it can still be quite mysterious, and, most recently, I've been >> trying to understand a bit more about the incremental marking part of the >> garbage collector. >> >> I'm working on the server for a competitive online game using node.js >> (primarily version 8.11.1 with v8 version 6.2.414.50). The server updates >> the game state around 100 times a second, and normally takes less than 1ms >> per update. Occasionally, though, we were seeing 16-50ms pauses between >> frames, which temporarily degraded the player experience. The first suspect >> was the garbage collector, but on my initial (incorrect) read, I didn't see >> anything taking over a few milliseconds. This log of a mark-sweep pass >> appeared to me to only take 2.1ms, for instance: >> >> [2944:0x356a660] 76591 ms: Mark-sweep 20.2 (40.9) -> 12.5 (40.9) MB, 2.1 >> / 0.0 ms (+ 26.1 ms in 18 steps since start of marking, biggest step 2.3 >> ms, walltime since start of marking 29 ms) finalize incremental marking >> via task GC in old space requested >> >> After much confusion, I eventually stumbled on the >> --trace_incremental_marking flag, which gave the following output: >> >> [2944:0x356a660] 76562 ms: [IncrementalMarking] Start (idle task): old >> generation 19MB, limit 27MB, slack 8MB >> [2944:0x356a660] 76562 ms: [IncrementalMarking] Start marking >> [2944:0x356a660] 76562 ms: [IncrementalMarking] Running >> [2944:0x356a660] 76564 ms: [IncrementalMarking] Step in task 827176 >> bytes (827100) in 1.9 >> [2944:0x356a660] 76566 ms: [IncrementalMarking] Step in task 827136 >> bytes (827100) in 1.9 >> [2944:0x356a660] 76567 ms: [IncrementalMarking] Step in task 827216 >> bytes (827100) in 1.1 >> [2944:0x356a660] 76569 ms: [IncrementalMarking] Step in task 878480 >> bytes (827100) in 1.5 >> [2944:0x356a660] 76570 ms: [IncrementalMarking] Step in task 827168 >> bytes (827100) in 1.1 >> [2944:0x356a660] 76571 ms: [IncrementalMarking] Step in task 827184 >> bytes (827100) in 1.0 >> [2944:0x356a660] 76573 ms: [IncrementalMarking] Step in task 849536 >> bytes (827100) in 1.7 >> [2944:0x356a660] 76575 ms: [IncrementalMarking] Step in task 827104 >> bytes (827100) in 2.2 >> [2944:0x356a660] 76576 ms: [IncrementalMarking] Step in task 857840 >> bytes (827100) in 1.1 >> [2944:0x356a660] 76577 ms: [IncrementalMarking] Step in task 1038576 >> bytes (827100) in 0.8 >> [2944:0x356a660] 76578 ms: [IncrementalMarking] Step in task 827160 >> bytes (827100) in 0.7 >> [2944:0x356a660] 76580 ms: [IncrementalMarking] Step in task 827128 >> bytes (827100) in 2.0 >> [2944:0x356a660] 76582 ms: [IncrementalMarking] Step in task 827112 >> bytes (827100) in 2.2 >> [2944:0x356a660] 76585 ms: [IncrementalMarking] Step in task 827104 >> bytes (827100) in 2.2 >> [2944:0x356a660] 76586 ms: [IncrementalMarking] Step in task 829880 >> bytes (827100) in 1.0 >> [2944:0x356a660] 76587 ms: [IncrementalMarking] Step in task 971384 >> bytes (827100) in 1.3 >> [2944:0x356a660] 76588 ms: [IncrementalMarking] requesting finalization >> of incremental marking. >> [2944:0x356a660] 76588 ms: [IncrementalMarking] Step in task 199296 >> bytes (827100) in 0.6 >> [2944:0x356a660] 76588 ms: [IncrementalMarking] (finalize incremental >> marking via task). >> [2944:0x356a660] 76588 ms: [IncrementalMarking] Finalize incrementally >> round 0, spent 0 ms, marking progress 13. >> [2944:0x356a660] 76588 ms: [IncrementalMarking] Black allocation started >> [2944:0x356a660] 76589 ms: [IncrementalMarking] Complete (normal). >> [2944:0x356a660] 76589 ms: [IncrementalMarking] Step in task 41240 bytes >> (827100) in 0.8 >> [2944:0x356a660] 76589 ms: [IncrementalMarking] Stopping: old generation >> 19MB, limit 27MB, overshoot 0MB >> [2944:0x356a660] 76589 ms: [IncrementalMarking] Black allocation finished >> >> ... revealing that incremental marking steps occur consecutively (and, in >> this case, blocked execution for 29ms). Since I'd read that incremental >> marking was designed to reduce jank when scrolling web pages, I'd assumed >> the marking steps would be spaced over time. After quite a lot of >> profiling, however, I've never seen control get returned to the program >> between the start of incremental marking and completion. >> >> We were eventually able to work around this by avoiding triggering any >> mark-sweeps during rounds of gameplay, but I am curious if this is expected >> behavior? Am I missing some sort of flag to tune the priority of the >> incremental marker? Game developers are used to working around blocking >> garbage collectors like this, so it's not too big an issue, but it would be >> a nice bonus if you didn't have to worry about it, and it feels like v8 is >> pretty close to getting there! >> >> Also, apologies if this is more of a node issue. If so, I can take my >> question over there. >> > -- > -- > v8-users mailing list > v8-users@googlegroups.com > http://groups.google.com/group/v8-users > --- > You received this message because you are subscribed to the Google Groups > "v8-users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to v8-users+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- -- v8-users mailing list v8-users@googlegroups.com http://groups.google.com/group/v8-users --- You received this message because you are subscribed to the Google Groups "v8-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to v8-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
