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SubjectRe: [PATCH RFC 2/2] futex: Implement mechanism to wait on any of several futexes
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.

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


> Thanks,
> tglx

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