Messages in this thread | | | Date | Sat, 7 May 2016 10:47:38 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [patch V2 3/7] futex: Add op for hash preallocation |
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On Fri, 6 May 2016, Darren Hart 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. > > "it's" or preferably "it is" > > > > > Add a futex op which allows to preallocate the hash with the requested > > "allows for preallocating" > > > size. The size is limited by the systemwide maximum hash size, which can be > > system-wide > > > 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. > > So this seems like it could be tricky for the user as system libraries, like > glibc, make use of futexes. Can we guarantee that "sys_futex" is not called by > the time main() is called?
To the extent of my testing I never observed that the hash was automatically created when I called futex(PREALLOC) right away in main. But yes, that might need some thought.
Thanks,
tglx
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