Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLEhandler

[Raghavendra K T]

On 10/11/2012 01:06 AM, Andrew Theurer wrote:
> On Wed, 2012-10-10 at 23:24 +0530, Raghavendra K T wrote:
> > On 10/10/2012 08:29 AM, Andrew Theurer wrote:
> > > On Wed, 2012-10-10 at 00:21 +0530, Raghavendra K T wrote:
> > > > * Avi Kivity <avi@xxxxxxxxxx> [2012-10-04 17:00:28]:
> > > >
> > > > > On 10/04/2012 03:07 PM, Peter Zijlstra wrote:
> > > > > > On Thu, 2012-10-04 at 14:41 +0200, Avi Kivity wrote:
> > > > > > >
[...]
> > > A big concern I have (if this is 1x overcommit) for ebizzy is that it
> > > has just terrible scalability to begin with.  I do not think we should
> > > try to optimize such a bad workload.
> >
> > I think my way of running dbench has some flaw, so I went to ebizzy.
> > Could you let me know how you generally run dbench?
>
> I mount a tmpfs and then specify that mount for dbench to run on.  This
> eliminates all IO.  I use a 300 second run time and number of threads is
> equal to number of vcpus.  All of the VMs of course need to have a
> synchronized start.
>
> I would also make sure you are using a recent kernel for dbench, where
> the dcache scalability is much improved.  Without any lock-holder
> preemption, the time in spin_lock should be very low:
>
>
> >      21.54%      78016         dbench  [kernel.kallsyms]   [k] copy_user_generic_unrolled
> >       3.51%      12723         dbench  libc-2.12.so        [.] __strchr_sse42
> >       2.81%      10176         dbench  dbench              [.] child_run
> >       2.54%       9203         dbench  [kernel.kallsyms]   [k] _raw_spin_lock
> >       2.33%       8423         dbench  dbench              [.] next_token
> >       2.02%       7335         dbench  [kernel.kallsyms]   [k] __d_lookup_rcu
> >       1.89%       6850         dbench  libc-2.12.so        [.] __strstr_sse42
> >       1.53%       5537         dbench  libc-2.12.so        [.] __memset_sse2
> >       1.47%       5337         dbench  [kernel.kallsyms]   [k] link_path_walk
> >       1.40%       5084         dbench  [kernel.kallsyms]   [k] kmem_cache_alloc
> >       1.38%       5009         dbench  libc-2.12.so        [.] memmove
> >       1.24%       4496         dbench  libc-2.12.so        [.] vfprintf
> >       1.15%       4169         dbench  [kernel.kallsyms]   [k] __audit_syscall_exit

Hi Andrew,
I ran the test with dbench with tmpfs. I do not see any improvements in
dbench for 16k ple window.

So it seems apart from ebizzy no workload benefited by that. and I
agree that, it may not be good to optimize for ebizzy.
I shall drop changing to 16k default window and continue with other
original patch series. Need to experiment with latest kernel.

(PS: Thanks for pointing towards, perf in latest kernel. It works fine.)

Results:
dbench run for 120 sec 30 sec warmup 8 iterations using tmpfs
base = 3.6.0-rc5 with ple handler optimization patch.

x => base + ple_window = 4k
+ => base + ple_window = 16k
* => base + ple_gap = 0

dbench 1x overcommit case
=========================
    N           Min           Max        Median           Avg        Stddev
x   8        5322.5       5519.05       5482.71     5461.0962     63.522276
+   8       5255.45       5530.55       5496.94     5455.2137     93.070363
*   8       5350.85       5477.81      5408.065     5418.4338     44.762697


dbench 2x overcommit case
==========================

    N           Min           Max        Median           Avg        Stddev
x   8       3054.32       3194.47       3137.33      3132.625     54.491615
+   8        3040.8       3148.87      3088.615     3088.1887     32.862336
*   8       3031.51       3171.99        3083.6     3097.4612     50.526977

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