Date: Tue, 10 Apr 2012 11:19:39 -0500 From: Alan Cox <alc@rice.edu> To: John Baldwin <jhb@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-hackers@freebsd.org, Andrey Zonov <andrey@zonov.org>, alc@freebsd.org Subject: Re: problems with mmap() and disk caching Message-ID: <4F845D9B.10004@rice.edu> In-Reply-To: <201204091126.25260.jhb@freebsd.org> References: <4F7B495D.3010402@zonov.org> <20120404071746.GJ2358@deviant.kiev.zoral.com.ua> <4F7DC037.9060803@rice.edu> <201204091126.25260.jhb@freebsd.org>
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On 04/09/2012 10:26, John Baldwin wrote: > 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? > Not yet. I did some tests with a file that was several times larger than DRAM, and I didn't like what I saw. Initially, everything behaved as expected, but about halfway through the test the bulk of the pages were active. Despite the call to pmap_clear_reference() in vm_page_dontneed(), the page daemon is finding the pages to be referenced and reactivating them. The net result is that the time it takes to read the file (from a relatively fast SSD) goes up by about 12%. So, this still needs work. Alan
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