[stgt] [Scst-devel] [Iscsitarget-devel] ISCSI-SCST performance (with also IET and STGT data)

Vladislav Bolkhovitin vst at vlnb.net
Thu Apr 2 11:02:32 CEST 2009

Vladislav Bolkhovitin, on 04/02/2009 11:38 AM wrote:
> James Bottomley, on 04/02/2009 12:23 AM wrote:
>> On Wed, 2009-04-01 at 08:20 -0400, Ross Walker wrote:
>>> On Apr 1, 2009, at 2:29 AM, Bart Van Assche <bart.vanassche at gmail.com>  
>>> wrote:
>>>> On Tue, Mar 31, 2009 at 8:43 PM, Ross S. W. Walker
>>>> <RWalker at medallion.com> wrote:
>>>>> IET just needs to fix how it does it workload with CFQ which
>>>>> somehow SCST has overcome. Of course SCST tweaks the Linux kernel to
>>>>> gain some extra speed.
>>>> I'm not familiar with the implementation details of CFQ, but I know
>>>> that one of the changes between SCST 1.0.0 and SCST 1.0.1 is that the
>>>> default number of kernel threads of the scst_vdisk kernel module has
>>>> been increased to 5. Could this explain the performance difference
>>>> between SCST and IET for FILEIO and BLOCKIO ?
>>> Thank for the update. IET has used 8 threads per target for ages now,  
>>> I don't think it is that.
>>> It may be how the I/O threads are forked in SCST that causes them to  
>>> be in the same I/O context with each other.
>>> I'm pretty sure implementing a version of the patch that was used for  
>>> the dump command (found on the LKML) will fix this.
>>> But thanks goes to Vlad for pointing this dificiency out so we can fix  
>>> it to help make IET even better.
>> SCST explicitly fiddles with the io context to get this to happen.  It
>> has a hack to block to export alloc_io_context:
>> http://marc.info/?t=122893564800003
> Correct, although I wouldn't call it "fiddle", rather "grouping" ;)
> But that's not the only reason for good performance. Particularly, it 
> can't explain Bart's tmpfs results from the previous message, where the 
> majority of I/O done to/from RAM without any I/O scheduler involved. (Or 
> does I/O scheduler also involved with tmpfs?) Bart has 4GB RAM, if I 
> remember correctly, i.e. the test data set was 25% of RAM.

To remove any suspicions that I'm playing dirty games here I should note 
that in many cases I can't say what exactly is responsible for good SCST 
performance. I can say only something like "good design and 
implementation", but, I guess, it wouldn't be counted too much. 
SCST/iSCSI-SCST from the very beginning were designed and made with the 
best performance in mind and that has brought the result. Sorry, but at 
the moment I can't afford doing any "why it's so good?" kinds of 
investigations, because I have a lot more important things to do, like 
SCST procfs -> sysfs interface conversion.

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