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Re: [patch V2 3/7] futex: Add op for hash preallocation

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  Re: [patch V2 3/7] futex: Add op for hash preallocation Peter Zijlstra <peterz@infradead.org> - 2016-05-19 14:30 +0200
    Re: [patch V2 3/7] futex: Add op for hash preallocation Darren Hart <dvhart@infradead.org> - 2016-05-19 21:40 +0200
      Re: [patch V2 3/7] futex: Add op for hash preallocation Peter Zijlstra <peterz@infradead.org> - 2016-05-20 07:00 +0200

#1403709 — Re: [patch V2 3/7] futex: Add op for hash preallocation

On Thu, May 05, 2016 at 08:44:05PM -0000, Thomas Gleixner wrote:
> From: Sebastian Siewior <bigeasy@linutronix.de>
> 
> The per process hash is allocated on the fly at the first futex operation of a
> process. The size of the hash is determined by a system wide default setting
> controlled by the sys admin, This is suboptimal for RT applications and
> applications with pathological futex abuse,
> 
>  - For RT applications its important to allocate the per process hash before the
>    first futex operation to avoid the allocation on the first futex operation.
> 
>  - For pathological applications which use gazillions of futexes its useful to
>    allocate a hash greater than the default hash size.
> 
> Add a futex op which allows to preallocate the hash with the requested
> size. The size is limited by the systemwide maximum hash size, which can be
> set by the admin. The requested size is rounded up to the next order of 2.
> 
> The function can be called several times, but ony the first call results in a
> hash allocation of the requested size as there is no non-intrusive way to
> reallocate/rehash in a multithreaded application.
> 
> Note, that this call must be issued before the first futex operation in the
> process because that would automatically allocate the default sized hash.
> 
> The function returns the actual hash size or 0 if the global hash is used. The
> latter is the case on UP and in the rare case that the allocation failed and
> the global hash is used as a fallback.

OK, so no on-line rehashing possible?

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#1403937

On Thu, May 19, 2016 at 02:24:57PM +0200, Peter Zijlstra wrote:
> On Thu, May 05, 2016 at 08:44:05PM -0000, Thomas Gleixner wrote:
> > From: Sebastian Siewior <bigeasy@linutronix.de>
> > 
> > The per process hash is allocated on the fly at the first futex operation of a
> > process. The size of the hash is determined by a system wide default setting
> > controlled by the sys admin, This is suboptimal for RT applications and
> > applications with pathological futex abuse,
> > 
> >  - For RT applications its important to allocate the per process hash before the
> >    first futex operation to avoid the allocation on the first futex operation.
> > 
> >  - For pathological applications which use gazillions of futexes its useful to
> >    allocate a hash greater than the default hash size.
> > 
> > Add a futex op which allows to preallocate the hash with the requested
> > size. The size is limited by the systemwide maximum hash size, which can be
> > set by the admin. The requested size is rounded up to the next order of 2.
> > 
> > The function can be called several times, but ony the first call results in a
> > hash allocation of the requested size as there is no non-intrusive way to
> > reallocate/rehash in a multithreaded application.
> > 
> > Note, that this call must be issued before the first futex operation in the
> > process because that would automatically allocate the default sized hash.
> > 
> > The function returns the actual hash size or 0 if the global hash is used. The
> > latter is the case on UP and in the rare case that the allocation failed and
> > the global hash is used as a fallback.
> 
> OK, so no on-line rehashing possible?

It doesn't do it currently... did you see something that makes it impossible to
add?


-- 
Darren Hart
Intel Open Source Technology Center

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#1404129

On Thu, May 19, 2016 at 12:38:19PM -0700, Darren Hart wrote:
> On Thu, May 19, 2016 at 02:24:57PM +0200, Peter Zijlstra wrote:
> > OK, so no on-line rehashing possible?
> 
> It doesn't do it currently... did you see something that makes it impossible to
> add?

No, just tons of tricky.

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