Thanks for the tips! I did some investigating and found that it doesn't look like JIT is a major driver here. Instead, it seems to be regular old memory allocation and deallocation. I think I can reduce the number of mmap ops considerably with a custom PageAllocator that batches operations and eliminates redundant ones (e.g. freeing some pages and then immediately allocating them again, or freeing pages on isolate shutdown that would be freed together with the whole pointer cage anyway).
We also noticed that freeing pages does two operations: mprotect(kNoAccess) followed by madvise(MADV_DONTNEED). It seems like we can get away with skipping the mprotect operation since V8 won't attempt to access the pages anyway, and the madvise operation is what actually frees the memory. This change alone looks like it may reduce mmap operations by ~30%-40%. -Kenton On Mon, Aug 8, 2022 at 5:23 AM Clemens Backes <cleme...@chromium.org> wrote: > If you are also executing WebAssembly, then > --no-wasm-write-protect-code-memory will further reduce the number of > mprotect calls. > We plan to turn off that feature anyway when we enable lazy compilation > for Wasm, so you won't lose much if you already disable it now. > > On Mon, Aug 8, 2022 at 12:04 PM 'Hannes Payer' via v8-dev < > v8-dev@googlegroups.com> wrote: > >> Hi Kenton, >> >> You can turn off the write protection for code memory with >> --nowrite_protect_code_memory. The security benefits of this feature are >> quite small. >> >> -Hannes >> >> On Fri, Aug 5, 2022 at 4:44 PM 'Kenton Varda' via v8-dev < >> v8-dev@googlegroups.com> wrote: >> >>> Hi v8-dev, >>> >>> In Cloudflare Workers, where we often have thousands of isolates running >>> across many threads in one process, we're running into issues with >>> contention on the process's mmap_lock / mmap_sem in the Linux kernel. It >>> appears V8 does a very large number of mmap/munmap/mprotect/page fault >>> operations and in the right circumstances this is enough to bog down the >>> lock. >>> >>> Before diving deep into the code I wanted to check if anyone has any >>> thoughts, off the top of your heads, on how we could reduce the mmap usage >>> in V8. Is there any easy knob to turn? Good places to look for >>> optimizations in the code? >>> >>> Assuming there isn't an easy answer, one idea suggested to me by a >>> kernel developer friend is: Instead of using mprotect() to swap pages >>> between write and execute mode while JITing, what if the pages were mapped >>> from a memfd in read-execute mode, and writes were performed by writing to >>> a separate buffer and then doing a pwrite() on the memfd? Does that sound >>> worth exploring? (I'm suggesting this with very little understanding of how >>> JIT works in V8; apologies if it's an obviously bad idea.) >>> >>> Thanks for any thoughts! >>> >>> -Kenton >>> >>> -- >>> -- >>> v8-dev mailing list >>> v8-dev@googlegroups.com >>> http://groups.google.com/group/v8-dev >>> --- >>> You received this message because you are subscribed to the Google >>> Groups "v8-dev" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to v8-dev+unsubscr...@googlegroups.com. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/v8-dev/109e3dbe-edbd-4aeb-a30a-24b932939becn%40googlegroups.com >>> <https://groups.google.com/d/msgid/v8-dev/109e3dbe-edbd-4aeb-a30a-24b932939becn%40googlegroups.com?utm_medium=email&utm_source=footer> >>> . >>> >> >> >> -- >> >> >> Hannes Payer | V8 | Google Germany GmbH | Erika-Mann Str. 33, 80636 >> München >> >> Geschäftsführer: Paul Manicle, Liana Sebastian >> >> Registergericht und -nummer: Hamburg, HRB 86891 >> >> Sitz der Gesellschaft: Hamburg >> >> Diese E-Mail ist vertraulich. Falls Sie diese fälschlicherweise erhalten >> haben sollten, leiten Sie diese bitte nicht an jemand anderes weiter, >> löschen Sie alle Kopien und Anhänge davon und lassen Sie mich bitte wissen, >> dass die E-Mail an die falsche Person gesendet wurde. >> >> >> >> This e-mail is confidential. If you received this communication by >> mistake, please don't forward it to anyone else, please erase all copies >> and attachments, and please let me know that it has gone to the wrong >> person. >> >> -- >> -- >> v8-dev mailing list >> v8-dev@googlegroups.com >> http://groups.google.com/group/v8-dev >> --- >> You received this message because you are subscribed to the Google Groups >> "v8-dev" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to v8-dev+unsubscr...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/v8-dev/CAKEgpyGJyiVCDNn2BN4TPpYqpxZFEGqh-JZb0oURSC2KnsiYAw%40mail.gmail.com >> <https://groups.google.com/d/msgid/v8-dev/CAKEgpyGJyiVCDNn2BN4TPpYqpxZFEGqh-JZb0oURSC2KnsiYAw%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> > > > -- > > Clemens Backes > > Software Engineer > > cleme...@google.com > > Google Germany GmbH > > Erika-Mann-Straße 33 > > 80636 München > > Geschäftsführer: Paul Manicle, Liana Sebastian > > Registergericht und -nummer: Hamburg, HRB 86891 > > Sitz der Gesellschaft: Hamburg > > Diese E-Mail ist vertraulich. Falls sie diese fälschlicherweise erhalten > haben sollten, leiten Sie diese bitte nicht an jemand anderes weiter, > löschen Sie alle Kopien und Anhänge davon und lassen Sie mich bitte wissen, > dass die E-Mail an die falsche Person gesendet wurde. > > > This e-mail is confidential. If you received this communication by > mistake, please don't forward it to anyone else, please erase all copies > and attachments, and please let me know that it has gone to the wrong > person. > > > -- > -- > v8-dev mailing list > v8-dev@googlegroups.com > http://groups.google.com/group/v8-dev > --- > You received this message because you are subscribed to a topic in the > Google Groups "v8-dev" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/v8-dev/OvqzZHemtDI/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > v8-dev+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/v8-dev/CAGO%3DqhCXJLfy9DMJ_Wh1Bcs1AS%3DiBjRKfTr0K60O-%3D6QQpxBmw%40mail.gmail.com > <https://groups.google.com/d/msgid/v8-dev/CAGO%3DqhCXJLfy9DMJ_Wh1Bcs1AS%3DiBjRKfTr0K60O-%3D6QQpxBmw%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- -- v8-dev mailing list v8-dev@googlegroups.com http://groups.google.com/group/v8-dev --- You received this message because you are subscribed to the Google Groups "v8-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to v8-dev+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/v8-dev/CAJouXQn16VWxhk1%3Dbzg_aSThX1uHmCQHEYTAsBTrWiSAGY199w%40mail.gmail.com.
