From: Mel Gorman <mgorman@suse.de>
To: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Tejun Heo <tj@kernel.org>,
cgroups@vger.kernel.org
Subject: Re: [PATCH] mm, memcg: Optionally disable memcg by default using Kconfig
Date: Tue, 19 May 2015 17:04:04 +0100 [thread overview]
Message-ID: <20150519160404.GJ2462@suse.de> (raw)
In-Reply-To: <20150519154119.GI2462@suse.de>
On Tue, May 19, 2015 at 04:41:19PM +0100, Mel Gorman wrote:
> On Tue, May 19, 2015 at 05:27:10PM +0200, Michal Hocko wrote:
> > On Tue 19-05-15 16:13:02, Mel Gorman wrote:
> > [...]
> > > :ffffffff811c160f: je ffffffff811c1630 <mem_cgroup_try_charge+0x40>
> > > :ffffffff811c1611: xor %eax,%eax
> > > :ffffffff811c1613: xor %ebx,%ebx
> > > 1 1.7e-05 :ffffffff811c1615: mov %rbx,(%r12)
> > > 7 1.2e-04 :ffffffff811c1619: add $0x10,%rsp
> > > 1211 0.0203 :ffffffff811c161d: pop %rbx
> > > 5 8.4e-05 :ffffffff811c161e: pop %r12
> > > 5 8.4e-05 :ffffffff811c1620: pop %r13
> > > 1249 0.0210 :ffffffff811c1622: pop %r14
> > > 7 1.2e-04 :ffffffff811c1624: pop %rbp
> > > 5 8.4e-05 :ffffffff811c1625: retq
> > > :ffffffff811c1626: nopw %cs:0x0(%rax,%rax,1)
> > > 295 0.0050 :ffffffff811c1630: mov (%rdi),%rax
> > > 160703 2.6973 :ffffffff811c1633: mov %edx,%r13d
> >
> > Huh, what? Even if this was off by one and the preceding instruction has
> > consumed the time. This would be reading from page->flags but the page
> > should be hot by the time we got here, no?
> >
>
> I would have expected so but it's not the first time I've seen cases where
> examining the flags was a costly instruction. I suspect it's due to an
> ordering issue or more likely, a frequent branch mispredict that is being
> accounted for against this instruction.
>
Which is plausible as forward branches are statically predicted false but
in this particular load that could be a close to a 100% mispredict.
--
Mel Gorman
SUSE Labs
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next prev parent reply other threads:[~2015-05-19 16:04 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-19 10:40 Mel Gorman
2015-05-19 14:18 ` Johannes Weiner
2015-05-19 14:43 ` Mel Gorman
2015-05-19 15:15 ` Michal Hocko
2015-05-19 17:09 ` Ben Hutchings
2015-05-19 14:53 ` Michal Hocko
2015-05-19 15:12 ` Vlastimil Babka
2015-05-19 15:13 ` Mel Gorman
2015-05-19 15:25 ` Vlastimil Babka
2015-05-19 16:14 ` Johannes Weiner
2015-05-19 15:27 ` Michal Hocko
2015-05-19 15:41 ` Mel Gorman
2015-05-19 16:04 ` Mel Gorman [this message]
2015-05-19 19:32 ` Mel Gorman
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