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

[Vladislav Bolkhovitin <vst@vlnb.net>]

Wu Fengguang, on 02/16/2009 05:34 AM wrote:
> On Fri, Feb 13, 2009 at 11:08:25PM +0300, Vladislav Bolkhovitin wrote:
>> Wu Fengguang, on 02/13/2009 04:57 AM wrote:
>>> On Thu, Feb 12, 2009 at 09:35:18PM +0300, Vladislav Bolkhovitin wrote:
>>>> Sorry for such a huge delay. There were many other activities I had 
>>>> to  do before + I had to be sure I didn't miss anything.
>>>>
>>>> We didn't use NFS, we used SCST (http://scst.sourceforge.net) with   
>>>> iSCSI-SCST target driver. It has similar to NFS architecture, where N 
>>>>  threads (N=5 in this case) handle IO from remote initiators 
>>>> (clients)  coming from wire using iSCSI protocol. In addition, SCST 
>>>> has patch  called export_alloc_io_context (see   
>>>> http://lkml.org/lkml/2008/12/10/282), which allows for the IO threads 
>>>>  queue IO using single IO context, so we can see if context RA can   
>>>> replace grouping IO threads in single IO context.
>>>>
>>>> Unfortunately, the results are negative. We find neither any 
>>>> advantages  of context RA over current RA implementation, nor 
>>>> possibility for  context RA to replace grouping IO threads in single 
>>>> IO context.
>>>>
>>>> Setup on the target (server) was the following. 2 SATA drives grouped 
>>>> in  md RAID-0 with average local read throughput ~120MB/s ("dd 
>>>> if=/dev/zero  of=/dev/md0 bs=1M count=20000" outputs "20971520000 
>>>> bytes (21 GB)  copied, 177,742 s, 118 MB/s"). The md device was 
>>>> partitioned on 3  partitions. The first partition was 10% of space in 
>>>> the beginning of the  device, the last partition was 10% of space in 
>>>> the end of the device,  the middle one was the rest in the middle of 
>>>> the space them. Then the  first and the last partitions were exported 
>>>> to the initiator (client).  They were /dev/sdb and /dev/sdc on it 
>>>> correspondingly.
>>> Vladislav, Thank you for the benchmarks! I'm very interested in
>>> optimizing your workload and figuring out what happens underneath.
>>>
>>> Are the client and server two standalone boxes connected by GBE?
>> Yes, they directly connected using GbE.
>>
>>> When you set readahead sizes in the benchmarks, you are setting them
>>> in the server side? I.e. "linux-4dtq" is the SCST server?
>> Yes, it's the server. On the client all the parameters were left default.
>>
>>> What's the
>>> client side readahead size?
>> Default, i.e. 128K
>>
>>> It would help a lot to debug readahead if you can provide the
>>> server side readahead stats and trace log for the worst case.
>>> This will automatically answer the above questions as well as disclose
>>> the micro-behavior of readahead:
>>>
>>>         mount -t debugfs none /sys/kernel/debug
>>>
>>>         echo > /sys/kernel/debug/readahead/stats # reset counters
>>>         # do benchmark
>>>         cat /sys/kernel/debug/readahead/stats
>>>
>>>         echo 1 > /sys/kernel/debug/readahead/trace_enable
>>>         # do micro-benchmark, i.e. run the same benchmark for a short time
>>>         echo 0 > /sys/kernel/debug/readahead/trace_enable
>>>         dmesg
>>>
>>> The above readahead trace should help find out how the client side
>>> sequential reads convert into server side random reads, and how we can
>>> prevent that.
>> We will do it as soon as we have a free window on that system.
> 
> Thank you. For NFS, the client side read/readahead requests will be
> split into units of rsize which will be served by a pool of nfsd
> concurrently and possibly out of order. Does SCST have the same
> process? If so, what's the rsize value for your SCST benchmarks?

No, there is no such splitting in SCST. Client sees raw SCSI disks from 
server and what client sends is directly and in full size sent by the 
server to its backstorage using regular buffered read() 
(fd->f_op->aio_read() followed by 
wait_on_retry_sync_kiocb()/wait_on_sync_kiocb() to be precise).

Thanks,
Vlad