Messages in this thread | | | Date | Tue, 25 Nov 2008 19:25:58 +0800 | From | Wu Fengguang <> | Subject | Re: Slow file transfer speeds with CFQ IO scheduler in some cases |
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On Tue, Nov 25, 2008 at 02:02:45PM +0300, Vladislav Bolkhovitin wrote: > > > Wu Fengguang wrote: >> On Thu, Nov 13, 2008 at 09:54:39AM +0100, Jens Axboe wrote: >>> On Thu, Nov 13 2008, Wu Fengguang wrote: >>>> Hi all, >>>> >>>> //Sorry for being late. >>>> >>>> On Wed, Nov 12, 2008 at 08:02:28PM +0100, Jens Axboe wrote: >>>> [...] >>>>> I already talked about this with Jeff on irc, but I guess should post it >>>>> here as well. >>>>> >>>>> nfsd aside (which does seem to have some different behaviour skewing the >>>>> results), the original patch came about because dump(8) has a really >>>>> stupid design that offloads IO to a number of processes. This basically >>>>> makes fairly sequential IO more random with CFQ, since each process gets >>>>> its own io context. My feeling is that we should fix dump instead of >>>>> introducing a fair bit of complexity (and slowdown) in CFQ. I'm not >>>>> aware of any other good programs out there that would do something >>>>> similar, so I don't think there's a lot of merrit to spending cycles on >>>>> detecting cooperating processes. >>>>> >>>>> Jeff will take a look at fixing dump instead, and I may have promised >>>>> him that santa will bring him something nice this year if he does (since >>>>> I'm sure it'll be painful on the eyes). >>>> This could also be fixed at the VFS readahead level. >>>> >>>> In fact I've seen many kinds of interleaved accesses: >>>> - concurrently reading 40 files that are in fact hard links of one single file >>>> - a backup tool that splits a big file into 8k chunks, and serve the >>>> {1, 3, 5, 7, ...} chunks in one process and the {0, 2, 4, 6, ...} >>>> chunks in another one >>>> - a pool of NFSDs randomly serving some originally sequential read >>>> requests - now dump(8) seems to have some similar problem. >>>> >>>> In summary there have been all kinds of efforts on trying to >>>> parallelize I/O tasks, but unfortunately they can easily screw up the >>>> sequential pattern. It may not be easily fixable for many of them. >>>> >>>> It is however possible to detect most of these patterns at the >>>> readahead layer and restore sequential I/Os, before they propagate >>>> into the block layer and hurt performance. >>>> >>>> Vitaly, if that's what you need, I can try to prepare a patch for >>>> testing out. >>> It's not easy. To really fix it, you have to get that sequential RA >>> pattern from just the single process. As soon as you spread the IO >>> between processes (eg N-1 aren't just getting cache hits), then you may >>> run into trouble on the IO scheduler side. >> >> Yes, it's not easy(or possible) to tell from file->f_ra all those >> cooperative processes working on the same sequential stream, since >> they will have different file->f_ra instances. In the case of NFSD, >> the file->f_ra may well be all zeros. >> >> Another scheme is to detect the sequential pattern via looking up >> the page cache, which provides one single and consistent view of the >> pages recently accessed. That makes sequential detection possible. >> >> The cost will be one extra page cache lookup per random read. >> If it's not acceptable, the corresponding code could be disabled >> by default. > > I think, this should be the best and the simplest way to go. Since in > most case data from the cache should be later copied to user, one more > page cache lookup should be negligible.
After the initial proposal, two merits come to my mind to implement cooperative sequential I/O detection in the readahead code:
1) readahead can make larger(hence more efficient) I/O requests 2) the page cache lookup trick eliminates the overheads of extra rbtrees.
So I would definitely like to try it out :-)
Thank you, Fengguang
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