From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mel Gorman Subject: Re: [PATCH] mm, memcg: Optionally disable memcg by default using Kconfig Date: Tue, 19 May 2015 17:04:04 +0100 Message-ID: <20150519160404.GJ2462@suse.de> References: <20150519104057.GC2462@suse.de> <20150519141807.GA9788@cmpxchg.org> <20150519145340.GI6203@dhcp22.suse.cz> <20150519151302.GG2462@suse.de> <20150519152710.GK6203@dhcp22.suse.cz> <20150519154119.GI2462@suse.de> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20150519154119.GI2462@suse.de> Sender: owner-linux-mm@kvack.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Michal Hocko Cc: Johannes Weiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Tejun Heo , cgroups@vger.kernel.org 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 > > > :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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org