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

[Wu Fengguang <wfg@linux.intel.com>]

On Tue, Nov 25, 2008 at 02:41:47PM +0300, Vladislav Bolkhovitin wrote:
> Wu Fengguang wrote:
>> On Tue, Nov 25, 2008 at 01:59:53PM +0300, Vladislav Bolkhovitin wrote:
>>> 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.
>>> I believe this would be the most effective way to go, especially in 
>>> case  if data delivery path to the original client has its own 
>>> latency  depended from the amount of transferred data as it is in the 
>>> case of  remote NFS mount, which does synchronous sequential reads. 
>>> In this case  it is essential for performance to make both links 
>>> (local to the storage  and network to the client) be always busy and 
>>> transfer data  simultaneously. Since the reads are synchronous, the 
>>> only way to achieve  that is perform read ahead on the server 
>>> sufficient to cover the network  link latency. Otherwise you would 
>>> end up with only half of possible  throughput.
>>>
>>> However, from one side, server has to have a pool of 
>>> threads/processes  to perform well, but, from other side, current 
>>> read ahead code doesn't  detect too well that those threads/processes 
>>> are doing joint sequential  read, so the read ahead window gets 
>>> smaller, hence the overall read  performance gets considerably 
>>> smaller too.
>>>
>>>> Vitaly, if that's what you need, I can try to prepare a patch for testing out.
>>> I can test it with SCST SCSI target sybsystem (http://scst.sf.net). 
>>> SCST  needs such feature very much, otherwise it can't get full 
>>> backstorage  read speed. The maximum I can see is about ~80MB/s from 
>>> ~130MB/s 15K RPM  disk over 1Gbps iSCSI link (maximum possible is 
>>> ~110MB/s).
>>
>> Thank you very much!
>>
>> BTW, do you implicate that the SCSI system (or its applications) has
>> similar behaviors that the current readahead code cannot handle well?
>
> No. SCSI target subsystem is not the same as SCSI initiator subsystem,  
> which usually called simply SCSI (sub)system. SCSI target is a SCSI  
> server. It has the same amount of common with SCSI initiator as there  
> is, e.g., between Apache (HTTP server) and Firefox (HTTP client).

Got it. So the SCSI server will split&spread sequential IO of one
single file to cooperative threads? I'm trying to understand why the
proposed page cache context based readahead would help a SCSI server.

Thanks,
Fengguang