Date: Tue, 05 Jul 2016 10:19:28 -0700 From: Matthew Macy <mmacy@nextbsd.org> To: "Karl Denninger" <karl@denninger.net> Cc: "" <freebsd-hackers@freebsd.org> Subject: Re: ZFS ARC and mmap/page cache coherency question Message-ID: <155bc1260e6.12001bf18198857.6272515207330027022@nextbsd.org> In-Reply-To: <7e00af5a-86cd-25f8-a4c6-2d946b507409@denninger.net> References: <20160630140625.3b4aece3@splash.akips.com> <CALXu0UfxRMnaamh%2Bpo5zp=iXdNUNuyj%2B7e_N1z8j46MtJmvyVA@mail.gmail.com> <20160703123004.74a7385a@splash.akips.com> <155afb8148f.c6f5294d33485.2952538647262141073@nextbsd.org> <45865ae6-18c9-ce9a-4a1e-6b2a8e44a8b2@denninger.net> <155b84da0aa.ad3af0e6139335.8627172617037605875@nextbsd.org> <7e00af5a-86cd-25f8-a4c6-2d946b507409@denninger.net>
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---- On Mon, 04 Jul 2016 19:26:06 -0700 Karl Denninger <karl@denninger.net> wrote ---- > > > On 7/4/2016 18:45, Matthew Macy wrote: > > > > > > ---- On Sun, 03 Jul 2016 08:43:19 -0700 Karl Denninger <karl@denninger.net> wrote ---- > > > > > > On 7/3/2016 02:45, Matthew Macy wrote: > > > > > > > > Cedric greatly overstates the intractability of resolving it. Nonetheless, since the initial import very little has been done to improve integration, and I don't know of anyone who is up to the task taking an interest in it. Consequently, mmap() performance is likely "doomed" for the foreseeable future.-M---- > > > > > > Wellllll.... > > > > > > I've done a fair bit of work here (see > > > https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=187594) and the > > > political issues are at least as bad as the coding ones. > > > > > > > > > Strictly speaking, the root of the problem is the ARC. Not ZFS per se. Have you ever tried disabling MFU caching to see how much worse LRU only is? I'm not really convinced the ARC's benefits justify its cost. > > > > -M > > > > The ARC is very useful when it gets a hit as it avoid an I/O that would > otherwise take place. > > Where it sucks is when the system evicts working set to preserve ARC. > That's always wrong in that you're trading a speculative I/O (if the > cache is hit later) for a *guaranteed* one (to page out) and maybe *two* > (to page back in.) The question wasn't ARC vs. no-caching. It was LRU only vs LRU + MFU. There are a lot of issues stemming from the fact that ZFS is a transactional object store with a POSIX FS on top. One is that it caches disk blocks as opposed to file blocks. However, if one could resolve that and have the page cache manage these blocks life would be much much better. However, you'd lose MFU. Hence my question. -M
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