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Subject: Re: [PATCH RFC 2/2] futex: Implement mechanism to wait on any of
 several futexes
To:     Thomas Gleixner <tglx@linutronix.de>,
        Gabriel Krisman Bertazi <krisman@collabora.com>
Cc:     mingo@redhat.com, peterz@infradead.org, dvhart@infradead.org,
        linux-kernel@vger.kernel.org, kernel@collabora.com,
        Steven Noonan <steven@valvesoftware.com>,
        "Pierre-Loup A . Griffais" <pgriffais@valvesoftware.com>
References: <20190730220602.28781-1-krisman@collabora.com>
 <20190730220602.28781-2-krisman@collabora.com>
 <alpine.DEB.2.21.1908010039470.1788@nanos.tec.linutronix.de>
From:   Zebediah Figura <z.figura12@gmail.com>
Message-ID: <31ad0ada-ecc7-60b3-e204-898460254be3@gmail.com>
Date:   Wed, 31 Jul 2019 20:22:54 -0500
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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 
clarification.

[1] 
https://github.com/ValveSoftware/wine/blob/proton_4.11/dlls/ntdll/fsync.c


> 
> Thanks,
> 
> 	tglx
>