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

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] Writing to a ramdisk in a PV domain is SLLLOOOWWW?!?
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 06 May 2010 10:42:53 -0700
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On 05/05/2010 11:14 AM, Dan Magenheimer wrote:
> 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 ??

I haven't looked at ramdisk, but I'm pretty sure there's nothing special
about accessing it.  The only thing I can think of is that if you're
using a 32bit highmem system then you may be being hit by lots of kmap
overhead.  But on a 64-bit system, AFAIK, it should just be memory copies.


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