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From: Zebediah Figura <>
To: Thomas Gleixner <>,
	Gabriel Krisman Bertazi <>
	Steven Noonan <>,
	"Pierre-Loup A . Griffais" <>
Subject: Re: [PATCH RFC 2/2] futex: Implement mechanism to wait on any of several futexes
Date: Wed, 31 Jul 2019 20:22:54 -0500
Message-ID: <> (raw)
In-Reply-To: <>

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

  reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-30 22:06 [PATCH RFC 1/2] futex: Split key setup from key queue locking and read Gabriel Krisman Bertazi
2019-07-30 22:06 ` [PATCH RFC 2/2] futex: Implement mechanism to wait on any of several futexes Gabriel Krisman Bertazi
2019-07-31 12:06   ` Peter Zijlstra
2019-07-31 15:15     ` Zebediah Figura
2019-07-31 22:39       ` Thomas Gleixner
2019-07-31 23:02         ` Zebediah Figura
2019-08-06  6:26     ` Gabriel Krisman Bertazi
2019-08-06 10:13       ` Peter Zijlstra
2019-08-01  0:45   ` Thomas Gleixner
2019-08-01  1:22     ` Zebediah Figura [this message]
2019-08-01  1:32       ` Zebediah Figura
2019-08-01  1:42         ` Pierre-Loup A. Griffais
2019-07-31 23:33 ` [PATCH RFC 1/2] futex: Split key setup from key queue locking and read Thomas Gleixner
2019-08-01  0:07   ` Gabriel Krisman Bertazi
2019-08-01  0:22     ` Gabriel Krisman Bertazi
2019-08-01  0:41     ` Thomas Gleixner

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