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RE: [Xen-devel] 10 million cycles disappearing

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, jbeulich@xxxxxxxxxx
Subject: RE: [Xen-devel] 10 million cycles disappearing
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Fri, 10 Apr 2009 06:20:35 -0700 (PDT)
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> >P.S. Average of tmem ops including page copy and
> >compression/decompresion is in the 20K-50K cycle range.
> How frequent is your tmem ops in a normal and extreme
> usage?

It's really hard to answer that.  There is no "normal"
usage as tmem gets used when there is memory pressure.
I stress-test it continually running "make -j80" on
linux-2.6.28 (usually with memory=768M and maxmem=1792M)
and with that load there are a few thousand tmem ops
per second.

Note also that the 20K-50K cycle range is WITH compression.
Compression is optional, slows the tmem op by 5x-10x,
but increases memory usage by 2x-4x.

When a VM is under memory pressure, it is often doing
I/O so CPU utilization is low.  Tmem absorbs some of the
otherwise unutilized CPU cycles and underutilized memory
to reduce I/O.

Does that help?

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