Re: [Scst-devel] [Iscsitarget-devel] ISCSI-SCST performance (withalso IET and STGT data)

[Vladislav Bolkhovitin]

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@xxxxxxxxx>  
> > > wrote:
> > >
> > > > On Tue, Mar 31, 2009 at 8:43 PM, Ross S. W. Walker
> > > > <RWalker@xxxxxxxxxxxxx> 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.

Thanks,
Vlad
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