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SubjectRe: [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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