Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751684Ab0DGEmP (ORCPT ); Wed, 7 Apr 2010 00:42:15 -0400 Received: from 207-172-69-77.c3-0.smr-ubr3.sbo-smr.ma.static.cable.rcn.com ([207.172.69.77]:47744 "EHLO thaum.luto.us" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751432Ab0DGEmK (ORCPT ); Wed, 7 Apr 2010 00:42:10 -0400 Message-ID: <4BBC0D1E.3030509@mit.edu> Date: Wed, 07 Apr 2010 00:42:06 -0400 From: Andy Lutomirski User-Agent: Thunderbird 2.0.0.24 (Windows/20100228) MIME-Version: 1.0 To: Peter Zijlstra CC: Suresh Jayaraman , LKML , Ingo Molnar Subject: Re: High priority threads causing severe CPU load imbalances References: <4BBB334D.5040308@suse.de> <1270562890.1595.438.camel@laptop> In-Reply-To: <1270562890.1595.438.camel@laptop> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4160 Lines: 87 Peter Zijlstra wrote: > On Tue, 2010-04-06 at 18:42 +0530, Suresh Jayaraman wrote: >> I have a simple test program that accepts number of threads(pthreads) to >> be created as a input. Each of these threads that gets created invokes a >> function which is just a infinite while loop. The main function after >> creating those threads goes in a infinite loop itself >> >> My test machine is a Dual Core AMD Opteron(tm) 860 with 8 >> sockets(non-HT), I run this test program with number of threads == >> number of CPUs: >> >> ./loadcpu -t 16 >> >> I see 100% CPU utilization on almost all CPUs (via mpstat/htop/vmstat). >> >> When the above threads are running, if I introduce a few high priority >> threads by doing: >> >> nice -n -13 ./loadcpu -t 3 >> >> After a short while, I see a few CPUs becoming idle at ~0% utilization >> (the number of CPUs becoming idle equals roughly the number of high >> priority threads i.e. 3). When I stop the high priority threads, the CPU >> utilization comes back to normal i.e. ~100%. >> >> This is reproducible on 2.6.32.10 stable kernel with all the recent all >> SMT fixes (I hope) and I think it would be reproducible in current >> upstream as well. > > Why bother using -stable for reporting bugs? > >> sched_mc_power_savings has been always set to 0. >> >> I spent a while staring at the load balancing and the thread migration >> code, but could not figure out why this is happening. Would appreciate >> any pointers. > > Right, except its not a severe imbalance as the subject suggests. For > some reason it seems to end up in a semi-stable state that is actually > quite balanced. > > for ((i=0; i<8; i++)) do while :; do :; done & done > for ((i=0; i<3; i++)) do while :; do :; done & renice -n -15 -p $! ; > done > > gets me: > > Cpu0 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st > Cpu1 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st > Cpu2 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st > Cpu3 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st > Cpu4 : 99.0%us, 1.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st > Cpu5 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st > Cpu6 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st > Cpu7 : 0.0%us, 0.0%sy, 0.0%ni,100.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st > Mem: 16440840k total, 1073672k used, 15367168k free, 105844k buffers > Swap: 16777212k total, 0k used, 16777212k free, 296504k cached > > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND > 4370 root 5 -15 105m 804 304 R 100.1 0.0 0:45.02 bash > 4374 root 5 -15 105m 804 304 R 100.1 0.0 0:44.95 bash > 4372 root 5 -15 105m 804 304 R 99.1 0.0 0:45.00 bash > 4364 root 20 0 105m 804 304 R 51.0 0.0 0:33.06 bash > 4362 root 20 0 105m 800 300 R 50.0 0.0 0:33.17 bash > 4365 root 20 0 105m 804 304 R 50.0 0.0 0:33.75 bash > 4368 root 20 0 105m 804 304 R 50.0 0.0 0:33.32 bash > 4369 root 20 0 105m 804 304 R 50.0 0.0 0:33.38 bash > 4363 root 20 0 105m 804 304 R 49.1 0.0 0:33.65 bash > 4366 root 20 0 105m 804 304 R 49.1 0.0 0:33.29 bash > 4367 root 20 0 105m 804 304 R 49.1 0.0 0:33.54 bash > > So we have the 3 -15 loops on a cpu each, and the 8 0 loops on 2 cpus > each, and 1 cpu idle. That is actually quite balanced, 'better' would be > if those 0 loops would rotate over the 5 available cpus, but that would > also trash more caches I guess. What's wrong with having the three -15 loops each get a CPU, having six of the remaining 0 loops get half a CPU, and the last two get their own CPUs. That's less fair but strictly better than the current solution, and nothing bounces. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/