On Fri, Jun 13, 2025 at 12:06 PM Erik Corry <erikco...@chromium.org> wrote:
> This isn't really caused by the IsolateGroup feature. The PagePool allows
> unused pages to be put in a pool so that other isolates can reuse them.
> This is a new performance feature from V8. It is more efficient to reuse a
> page than to unmap and then remap it later, which is two syscalls and
> probably disrupts the TLB.
>
> The page pool would cause the same inter-isolate interactions if
> IsolateGroups did not exist.
>
> I think the following untested uncompiled patch might fix your problem:
>
> diff --git a/src/heap/heap.cc b/src/heap/heap.cc
> index 090c1f780d2..d063998a963 100644
> --- a/src/heap/heap.cc
> +++ b/src/heap/heap.cc
> @@ -6402,6 +6402,7 @@ void Heap::TearDown() {
>
> read_only_space_ = nullptr;
>
> + memory_allocator()->pool()->ReleaseImmediately(isolate());
> memory_allocator()->pool()->ReleaseOnTearDown(isolate());
> memory_allocator()->TearDown();
>
>
> This will just release pages immediately, instead of putting them in the
> shared pool where other isolates can use them.
>
Yes, this will work. We will need to refactor the pool a bit in the near
term and can add a few flags to disable it selectively.
>
> On Thu, Jun 12, 2025 at 4:44 PM 'James Snell' via v8-dev <
> v8-dev@googlegroups.com> wrote:
>
>> Hey all,
>>
>> Yeah, we (Cloudflare Workers runtime folks) worked with Igalia and asked
>> them to implement the IsolateGroups mechanism specifically for pointer
>> compression support and would really have no intention of supporting it
>> without pointer compression. In workers we will create thousands of
>> isolates within a single process and can't afford to be limited by the
>> single pointer compression cage for the entire process. We also want to
>> start making use of the v8 sandbox. We were running a non-supported
>> configuration with pointer compression enabled but otherwise diverging from
>> the supported configuration in a way that was not sustainable and isolate
>> groups allow us to have better alignment there. It *might* be possible to
>> have a variation of isolate groups that works without pointer compression
>> but it's not something that we'd be interested in and not something we'd
>> ask our friends at Igalia to work on.
>>
>> - James
>>
>>
>> On Thu, Jun 12, 2025 at 7:10 AM Michael Lippautz <mlippa...@chromium.org>
>> wrote:
>>
>>>
>>>
>>> On Thu, Jun 12, 2025 at 3:37 PM ClearScript Developers <
>>> clearscript...@gmail.com> wrote:
>>>
>>>> Greetings!
>>>>
>>>> We've run into a new issue in V8 13.7 (upgrading from 13.5). In a
>>>> multi-isolate application, tearing down one isolate can trigger synchronous
>>>> activity in another – specifically, the posting of ReleasePooledChunksTask.
>>>>
>>>> Evidently, that happens because, by default, both isolates are in the
>>>> same group. Our understanding is that isolate groups are a new feature that
>>>> allows isolates to share certain resources, and that, unfortunately, is a
>>>> problem for us. In our case, isolates must remain... isolated.
>>>>
>>>> Setting up a dedicated group for each isolate appears to be possible,
>>>> but isolate groups require pointer compression, which we'd prefer to
>>>> disable. Even if we enabled it, pointer compression isn't supported on
>>>> 32-bit systems, which we still support.
>>>>
>>>> Can someone shed some light? Why do isolate groups require pointer
>>>> compression? How difficult would it be to remove that restriction?
>>>>
>>>>
>>> At this point V8 only officially supports a configuration with a single
>>> IsolateGroup.
>>>
>>> The concept of IsolateGroup was introduced by other embedders and as you
>>> wrote it's really for sharing a bunch of resources. E.g., read-only space,
>>> page pool, and at this point also a pointer compression cage. You could
>>> imagine an IsolateGroup without pointer compression -- at this point this
>>> is just not implemented.
>>>
>>> For maintenance and security reasons we can't accept any non-trivial
>>> patches for this area at this point as we cannot reliably test other
>>> configurations and ensure that they don't cause security problems down the
>>> line.
>>>
>>> -Michael
>>>
>>>
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