From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: Writing to a ramdisk in a PV domain is SLLLOOOWWW?!? Date: Thu, 06 May 2010 10:42:53 -0700 Message-ID: <4BE2FF9D.9010500@goop.org> References: <89f1cead-b77d-4294-ab12-5e05344ed346@default> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <89f1cead-b77d-4294-ab12-5e05344ed346@default> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Dan Magenheimer Cc: "Xen-Devel (xen-devel@lists.xensource.com)" List-Id: xen-devel@lists.xenproject.org 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. J