Date: Wed, 23 May 2012 10:02:14 +0400 From: Andrey Zonov <andrey@zonov.org> To: Alan Cox <alc@rice.edu> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-hackers@freebsd.org, alc@freebsd.org Subject: Re: problems with mmap() and disk caching Message-ID: <4FBC7D66.2080605@zonov.org> In-Reply-To: <4F9DD372.1020001@rice.edu> References: <4F7B495D.3010402@zonov.org> <20120404071746.GJ2358@deviant.kiev.zoral.com.ua> <4F7DC037.9060803@rice.edu> <201204091126.25260.jhb@freebsd.org> <4F845D9B.10004@rice.edu> <4F851F87.3050206@zonov.org> <4F9DD372.1020001@rice.edu>
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On 4/30/12 3:49 AM, Alan Cox wrote: > On 04/11/2012 01:07, Andrey Zonov wrote: >> On 10.04.2012 20:19, Alan Cox wrote: >>> 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. >>> >> >> Hi Alan, >> >> What do you think about attached patch? >> >> > > Sorry for the slow reply, I've been rather busy for the past couple of > weeks. What you propose is clearly good for sequential accesses, but not > so good for random accesses. Keep in mind, the potential costs of > unconditionally increasing the read window include not only wasted I/O > but also increased memory pressure. Rather than argue about which is > more important, sequential or random access, I think it's more > productive to replace the sequential access heuristic. The current > heuristic is just not that sophisticated. It's easy to do better. > > The attached patch implements a new heuristic, which starts with the > same initial read window as the current heuristic, but arithmetically > grows the window on sequential page faults. From a stylistic standpoint, > this patch also cleanly separates the "read ahead" logic from the "cache > behind" logic. > > At the same time, this new heuristic is more selective about performing > cache behind. It requires three or four sequential page faults before > cache behind is enabled. More precisely, it requires the read ahead > window to reach its maximum size before cache behind is enabled. > > For long, sequential accesses, the results of my performance tests are > just good as unconditionally increasing the window size. I'm also seeing > fewer pages needlessly cached by the cache behind heuristic. That said, > there is still room for improvement. We are still not achieving the same > sequential performance as "dd", and there are still more pages being > cached than I would like. > > Alan > > I've widely tested your patch and it showed good enough results. I've commited it in our tree and it will be soon on production cluster. Thanks a lot for help and your improvements! -- Andrey Zonov
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