Date: Fri, 5 Sep 2014 02:02:51 -0500 From: Alan Cox <alan.l.cox@gmail.com> To: Pieter de Goeje <pieter@degoeje.nl>, Konstantin Belousov <kostikbel@gmail.com> Cc: hackers@freebsd.org Subject: Re: mmap MAP_NOSYNC regression in 10.x Message-ID: <CAJUyCcNiLwLuL9crpQBjSdg4ED5kR53fPjyJG3HNmP5Roor8RQ@mail.gmail.com> In-Reply-To: <540903FF.6010602@degoeje.nl> References: <540903FF.6010602@degoeje.nl>
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On Thu, Sep 4, 2014 at 7:29 PM, Pieter de Goeje <pieter@degoeje.nl> wrote: > After upgrading my month old 10-stable installation today (to r271093) , > I've noticed a that the kernel no longer honors the MAP_NOSYNC flag. > Attached is a demonstration program that highlights the issue (also > available here: http://pastebin.com/y0kvdn0r ). > > The program creates and mmap()s a 200MiB file and repeatedly writes zeros > to it. The expected behavior is that under normal circumstances (no memory > pressure), the dirtied pages are not flushed to disk. Observed is however > that every ~30 seconds the syncer kicks in and basically halts the program > while it does its job. The program prints a line everytime the throughput > drops below 500MBps, well below memory bandwidth. > > mmap() is called like this: > > void *p = mmap(NULL, len, PROT_READ | PROT_WRITE, > MAP_SHARED | MAP_NOSYNC | MAP_ALIGNED_SUPER, fd, 0); > > Sample output: > > write... > zeroing: 209.6 MB > ...write: 5.839s > mmap... > ...mmap: 0.000s > 20.1s: memset #259: 34.7MBps - stalled > 55.7s: memset #810: 34.7MBps - stalled > 91.3s: memset #1359: 34.6MBps - stalled > 100.0s: memset #1522: 3938.5MBps > overall bandwidth: 3190.6MBps > munmap... > ...munmap: 5.796s > done > > (this is a rather old system) > > If necessary I'm willing to find out the exact commit that caused the > problem. > > That's not necessary. This is a bug in the page fault handler's new fast path. <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>;
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