RE: Writing to a ramdisk in a PV domain is SLLLOOOWWW?!?

[Dan Magenheimer <dan.magenheimer@oracle.com>]

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