ArrayBuffer BackingStore mutex causing high system load

[billywhizz]

Hi,

I have put some details here on an issue i have come across while doing some benchmarking: https://gist.github.com/billywhizz/ff4c83c37142198d2e70992e438bf045

I have a few questions around this:

1) Do we need a pthread_mutex_lock and pthread_mutex_unlock in every access to ArrayBuffer->GetBackingStore? In the absence of a deep dive into the code (which I will do soon hopefully) I am guessing this is in case the GC, which is running on another thread, has moved the backing data elsewhere.

2) If i want to access backing data at high frequency from JS (e.g. doing lots of writing or reading of an arraybuffer on a socket) is it possible (or advisable) to do something like this in order to avoid the lock contention in GetBackingStore? https://gist.github.com/billywhizz/ff4c83c37142198d2e70992e438bf045#file-test-cc. i.e. cache the pointer to the backing data in an internal field, assuming it will not be moved.

3) If the approach in 2) is not possible, is there a known approach to this problem which can avoid all the syscall overhead of locking the mutex?

If you need more info or something easy to reproduce please let me know.

Many Thanks,

Andrew

[Ulan Degenbaev]

Hi Andrew!

Could you please file a bug at crbug.com/v8 with steps to reproduce? We will take a look and see if we can optimize this case.

Until it is fixed, I think caching the backing store pointer in the internal field would work. Then you'd need to ensure that you keep a handle to ArrayBuffer while using the pointer (to prevent ArrayBuffer from being garbage collected). Additionally, the pointer may become invalid if ArrayBuffer is detached. You can detect that case by checking ByteLength() == 0. 

Cheers,

Ulan.

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[Andrew Johnston]

Hi Ulan,

Thanks for the response. I will do that over next couple days. It could be an edge case or could be related to something stupid I am doing when running isolates on threads in the same process (https://github.com/just-js/just/blob/main/just.cc#L2722). When i use separate processes I don't see the contention/syscall overhead. Will see if i can make a small and reproducible case for a bug report.

Andrew