Am 14.09.2013 um 00:24 schrieb Paul Mackerras <paulus@xxxxxxxxx>: > On Thu, Sep 12, 2013 at 06:01:37PM -0500, Alexander Graf wrote: >> >> On 05.08.2013, at 23:27, Paul Mackerras wrote: >> >>> Currently we request write access to all pages that get mapped into the >>> guest, even if the guest is only loading from the page. This reduces >>> the effectiveness of KSM because it means that we unshare every page we >>> access. Also, we always set the changed (C) bit in the guest HPTE if >>> it allows writing, even for a guest load. >> >> Have you measured how much performance we lose by mapping it twice? Usually Linux will mark user pages that are not written to yet as non-writable, no? That's why I assumed that "may_write" is the same as "guest wants to write" back when I wrote this. >> >> I'm also afraid that a sequence like >> >> ld x,y >> std x,y >> >> in the kernel will trap twice and slow us down heavily. But maybe I'm just being paranoid. Can you please measure bootup time with and without this, as well as a fork bomb (spawn /bin/echo 1000 times and time it) with and without so we get a feeling for its impact? > > Bootup (F19 guest, 3 runs): > > Without the patch: average 20.12 seconds, st. dev. 0.17 seconds > With the patch: 20.47 seconds, st. dev. 0.19 seconds > > Delta: 0.35 seconds, or 1.7%. > > time for i in $(seq 1000); do /bin/echo $i >/dev/null; done: > > Without the patch: average 7.27 seconds, st. dev. 0.23 seconds > With the patch: average 7.55 seconds, st. dev. 0.39 seconds > > Delta: 0.28 seconds, or 3.8%. > > So there appears to be a small effect, of a few percent. So in the normal case it slows us down, but allows ksm to be effective. Do we actually want this change then? Alex > > Paul. > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
