Messages in this thread | | | Date | Tue, 14 Jul 2015 14:17:04 -0700 | From | Dave Hansen <> | Subject | Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw_semaphore |
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On 07/14/2015 06:37 AM, Oleg Nesterov wrote: > On 07/14, Jan Kara wrote: >> So unless >> I'm missing something and there is a significant performance advantage to >> Dave's patches I'm all for using a generic primitive you suggest. > > I think percpu_rw_semaphore looks a bit better. And even a bit faster. > And it will not block __sb_start_write() entirely while freeze_super() > sleeps in synchronize_rcu().
That's true, but freeze_super() and the code blocked by it is a super-rare path compared with write().
> freeze_super() should be faster too after rcu_sync changes, but this > is not that important. > > But again, to me the main advantage is that we can use the generic > primitives and remove this nontrivial code in fs/super.c. > >> Can you perhaps work with Dave on some common resolution? > > Dave, what do you think? Will you agree with percpu_rw_semaphore ?
Using my little write-1-byte test (under will-it-scale), your 4 patches improves the number of writes/sec by 12%. My 3 patches improve the number of writes/sec by 32%.
My patches manage to get rid of the memory barriers entirely in the fast path. Your approach keeps the barriers.
Test: https://www.sr71.net/~dave/intel/write1byte.c
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