Date: Mon, 9 Apr 2012 11:26:25 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: Konstantin Belousov <kostikbel@gmail.com>, alc@freebsd.org, Andrey Zonov <andrey@zonov.org>, Alan Cox <alc@rice.edu> Subject: Re: problems with mmap() and disk caching Message-ID: <201204091126.25260.jhb@freebsd.org> In-Reply-To: <4F7DC037.9060803@rice.edu> References: <4F7B495D.3010402@zonov.org> <20120404071746.GJ2358@deviant.kiev.zoral.com.ua> <4F7DC037.9060803@rice.edu>
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On Thursday, April 05, 2012 11:54:31 am Alan Cox wrote: > On 04/04/2012 02:17, Konstantin Belousov wrote: > > On Tue, Apr 03, 2012 at 11:02:53PM +0400, Andrey Zonov wrote: > >> Hi, > >> > >> I open the file, then call mmap() on the whole file and get pointer, > >> then I work with this pointer. I expect that page should be only once > >> touched to get it into the memory (disk cache?), but this doesn't work! > >> > >> I wrote the test (attached) and ran it for the 1G file generated from > >> /dev/random, the result is the following: > >> > >> Prepare file: > >> # swapoff -a > >> # newfs /dev/ada0b > >> # mount /dev/ada0b /mnt > >> # dd if=/dev/random of=/mnt/random-1024 bs=1m count=1024 > >> > >> Purge cache: > >> # umount /mnt > >> # mount /dev/ada0b /mnt > >> > >> Run test: > >> $ ./mmap /mnt/random-1024 30 > >> mmap: 1 pass took: 7.431046 (none: 262112; res: 32; super: > >> 0; other: 0) > >> mmap: 2 pass took: 7.356670 (none: 261648; res: 496; super: > >> 0; other: 0) > >> mmap: 3 pass took: 7.307094 (none: 260521; res: 1623; super: > >> 0; other: 0) > >> mmap: 4 pass took: 7.350239 (none: 258904; res: 3240; super: > >> 0; other: 0) > >> mmap: 5 pass took: 7.392480 (none: 257286; res: 4858; super: > >> 0; other: 0) > >> mmap: 6 pass took: 7.292069 (none: 255584; res: 6560; super: > >> 0; other: 0) > >> mmap: 7 pass took: 7.048980 (none: 251142; res: 11002; super: > >> 0; other: 0) > >> mmap: 8 pass took: 6.899387 (none: 247584; res: 14560; super: > >> 0; other: 0) > >> mmap: 9 pass took: 7.190579 (none: 242992; res: 19152; super: > >> 0; other: 0) > >> mmap: 10 pass took: 6.915482 (none: 239308; res: 22836; super: > >> 0; other: 0) > >> mmap: 11 pass took: 6.565909 (none: 232835; res: 29309; super: > >> 0; other: 0) > >> mmap: 12 pass took: 6.423945 (none: 226160; res: 35984; super: > >> 0; other: 0) > >> mmap: 13 pass took: 6.315385 (none: 208555; res: 53589; super: > >> 0; other: 0) > >> mmap: 14 pass took: 6.760780 (none: 192805; res: 69339; super: > >> 0; other: 0) > >> mmap: 15 pass took: 5.721513 (none: 174497; res: 87647; super: > >> 0; other: 0) > >> mmap: 16 pass took: 5.004424 (none: 155938; res: 106206; super: > >> 0; other: 0) > >> mmap: 17 pass took: 4.224926 (none: 135639; res: 126505; super: > >> 0; other: 0) > >> mmap: 18 pass took: 3.749608 (none: 117952; res: 144192; super: > >> 0; other: 0) > >> mmap: 19 pass took: 3.398084 (none: 99066; res: 163078; super: > >> 0; other: 0) > >> mmap: 20 pass took: 3.029557 (none: 74994; res: 187150; super: > >> 0; other: 0) > >> mmap: 21 pass took: 2.379430 (none: 55231; res: 206913; super: > >> 0; other: 0) > >> mmap: 22 pass took: 2.046521 (none: 40786; res: 221358; super: > >> 0; other: 0) > >> mmap: 23 pass took: 1.152797 (none: 30311; res: 231833; super: > >> 0; other: 0) > >> mmap: 24 pass took: 0.972617 (none: 16196; res: 245948; super: > >> 0; other: 0) > >> mmap: 25 pass took: 0.577515 (none: 8286; res: 253858; super: > >> 0; other: 0) > >> mmap: 26 pass took: 0.380738 (none: 3712; res: 258432; super: > >> 0; other: 0) > >> mmap: 27 pass took: 0.253583 (none: 1193; res: 260951; super: > >> 0; other: 0) > >> mmap: 28 pass took: 0.157508 (none: 0; res: 262144; super: > >> 0; other: 0) > >> mmap: 29 pass took: 0.156169 (none: 0; res: 262144; super: > >> 0; other: 0) > >> mmap: 30 pass took: 0.156550 (none: 0; res: 262144; super: > >> 0; other: 0) > >> > >> If I ran this: > >> $ cat /mnt/random-1024> /dev/null > >> before test, when result is the following: > >> > >> $ ./mmap /mnt/random-1024 5 > >> mmap: 1 pass took: 0.337657 (none: 0; res: 262144; super: > >> 0; other: 0) > >> mmap: 2 pass took: 0.186137 (none: 0; res: 262144; super: > >> 0; other: 0) > >> mmap: 3 pass took: 0.186132 (none: 0; res: 262144; super: > >> 0; other: 0) > >> mmap: 4 pass took: 0.186535 (none: 0; res: 262144; super: > >> 0; other: 0) > >> mmap: 5 pass took: 0.190353 (none: 0; res: 262144; super: > >> 0; other: 0) > >> > >> This is what I expect. But why this doesn't work without reading file > >> manually? > > Issue seems to be in some change of the behaviour of the reserv or > > phys allocator. I Cc:ed Alan. > > I'm pretty sure that the behavior here hasn't significantly changed in > about twelve years. Otherwise, I agree with your analysis. > > On more than one occasion, I've been tempted to change: > > pmap_remove_all(mt); > if (mt->dirty != 0) > vm_page_deactivate(mt); > else > vm_page_cache(mt); > > to: > > vm_page_dontneed(mt); > > because I suspect that the current code does more harm than good. In > theory, it saves activations of the page daemon. However, more often > than not, I suspect that we are spending more on page reactivations than > we are saving on page daemon activations. The sequential access > detection heuristic is just too easily triggered. For example, I've > seen it triggered by demand paging of the gcc text segment. Also, I > think that pmap_remove_all() and especially vm_page_cache() are too > severe for a detection heuristic that is so easily triggered. Are you planning to commit this? -- John Baldwin
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