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
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