Re: [Xen-devel] Does Xen hypervisor overwrite O_DIRECT setting of Linux2.6 kernel?

["Liang Yang" <multisyncfe991@xxxxxxxxxxx>]

Yes, all the testing configurations are the exactly same (both uses Linux 
kernel 2.6.16.29) and I used Xen 3.0.3. I did several indepedent runs for my 
testing and the results are consistent, e.g. the performance of small packet 
sequential write under Xen Domain0 outperform Linux native by 10~20%.
Some people told me Xen hypervisor adds addtional layer beyond Linux I/O 
stack. So O_DIRECT under Xen hypervisor is still valid, but I/O coalescing 
for small packets are enabled by Xen hypervisor. However, Linux native 
kernel would not do any I/O coalescing when using O_DIRECT.
I hope Xen gurus can give me more explanations about this.

Thanks,

Liang

----- Original Message ----- 
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
To: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>; 
<xen-devel@xxxxxxxxxxxxxxxxxxx>
Cc: <ian.pratt@xxxxxxxxxxxx>
Sent: Saturday, October 28, 2006 2:34 PM
Subject: RE: [Xen-devel] Does Xen hypervisor overwrite O_DIRECT setting of 
Linux2.6 kernel?

> The performance data is collected on 8 SAS drives (used as physical
drives)
> and IOMeter is used as the benchmark tool. The latest IOMeter version
used
> O_DIRECT. We know, Linux 2.6 kernel starts supporting O_DIRECT which
makes
> all I/O requests work around buffer cache. The good thing for O_DIRECT
is
> it
> reduces the CPU utilization and cache pollution. The bad thing is
O_DIRECT
> not only forces all I/O requests become synchronous and no I/O
coalescing
> will happen. Thus sequential write of small packets will be impacted
most.
> For Xen, however, I believe Xen hypervisor overwrites this O_DIRECT
setting
> and maybe it favors better performance over CPU and FSB utilization.
Thus
> Xen domain0 can have better write performance than Linux native.
Are you sure you're comparing identical native and dom0 kernel versions?
Same drivers and settings?
Xen does not disable O_DIRECT.

Ian


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel