Re: Slow file transfer speeds with CFQ IO scheduler in some cases

["Vitaly V. Bursov" <vitalyb@telenet.dn.ua>]

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.

Deadline scheduler should fit my needs, I believe. I can test a patch which
tries to resolve the issue or run some more tests, though.

-- 
Thanks,
Vitaly