From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Magenheimer Subject: RE: Writing to a ramdisk in a PV domain is SLLLOOOWWW?!? Date: Wed, 5 May 2010 13:16:31 -0700 (PDT) Message-ID: References: <89f1cead-b77d-4294-ab12-5e05344ed346@default> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable 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: "Xen-Devel (xen-devel@lists.xensource.com)" List-Id: xen-devel@lists.xenproject.org It appears that ramswap is bad for HV domains also. On one run: real 11m38.017s user 0m6.696s sys 0m27.865s I tried on bare-metal Linux and there is a similar pattern but not nearly as bad: real 0m54.978s user 0m8.577s sys 0m3.094s Maybe there is some horrible TLB thrashing going on even on bare-metal that is made even worse by virtualization? > -----Original Message----- > From: Dan Magenheimer > Sent: Wednesday, May 05, 2010 12:14 PM > To: Xen-Devel (xen-devel@lists.xensource.com) > Subject: Writing to a ramdisk in a PV domain is SLLLOOOWWW?!? >=20 > Has anyone tried using a ramdisk in a PV domain? >=20 > I'm trying it in an experiment to compare swapping to > a ramdisk against tmem (frontswap) performance > (suggested as a performance test on lkml). >=20 > 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. >=20 > 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 ?? >=20 > Thanks, > Dan >=20 > P.S. Yes, I realize a ramdisk has no real value in > a guest... this is just a performance experiment.