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SubjectRe: [PATCH RFC 2/2] futex: Implement mechanism to wait on any of several futexes
On 7/31/19 8:22 PM, Zebediah Figura wrote:
> On 7/31/19 7:45 PM, Thomas Gleixner wrote:
>> If I assume a maximum of 65 futexes which got mentioned in one of the
>> replies then this will allocate 7280 bytes alone for the futex_q array with
>> a stock debian config which has no debug options enabled which would bloat
>> the struct. Adding the futex_wait_block array into the same allocation
>> becomes larger than 8K which already exceeds thelimit of SLUB kmem
>> caches and forces the whole thing into the page allocator directly.
>> This sucks.
>> Also I'm confused about the 64 maximum resulting in 65 futexes comment in
>> one of the mails.
>> Can you please explain what you are trying to do exatly on the user space
>> side?
> The extra futex comes from the fact that there are a couple of, as it
> were, out-of-band ways to wake up a thread on Windows. [Specifically, a
> thread can enter an "alertable" wait in which case it will be woken up
> by a request from another thread to execute an "asynchronous procedure
> call".] It's easiest for us to just add another futex to the list in
> that case.

To be clear, the 64/65 distinction is an implementation detail that's
pretty much outside the scope of this discussion. I should have just
said 65 directly. Sorry about that.

> I'd also point out, for whatever it's worth, that while 64 is a hard
> limit, real applications almost never go nearly that high. By far the
> most common number of primitives to select on is one.
> Performance-critical code never tends to wait on more than three. The
> most I've ever seen is twelve.
> If you'd like to see the user-side source, most of the relevant code is
> at [1], in particular the functions __fsync_wait_objects() [line 712]
> and do_single_wait [line 655]. Please feel free to ask for further
> clarification.
> [1]
>> Thanks,
>> tglx

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