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[Xen-devel] Writing to a ramdisk in a PV domain is SLLLOOOWWW?!?

To: "Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Writing to a ramdisk in a PV domain is SLLLOOOWWW?!?
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 5 May 2010 11:14:20 -0700 (PDT)
Delivery-date: Wed, 05 May 2010 11:16:38 -0700
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Has anyone tried using a ramdisk in a PV domain?

I'm trying it in an experiment to compare swapping to
a ramdisk against tmem (frontswap) performance
(suggested as a performance test on lkml).

Writing to the ramdisk appears to be VERY VERY slow,
elapsed time in the guest is several times larger than
user+sys, and xentop shows the guest consuming vcpu
seconds at about the user+sys rate.  Note that
this is when tmem is turned off and there is no
vhd swap disk configured.

I'm suspecting that writing to ramdisk must be causing
some interesting/expensive PV pagetable behavior?
Or maybe somehow /dev/ram0 is getting routed through
qemu?  Or ??


P.S. Yes, I realize a ramdisk has no real value in
a guest... this is just a performance experiment.

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