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Date: Wed, 21 Aug 2013 23:05:51 +0200 From: "Tibor Billes" <tbilles@....com> To: paulmck@...ux.vnet.ibm.com Cc: linux-kernel@...r.kernel.org Subject: Re: Unusually high system CPU usage with recent kernels > From: Paul E. McKenney Sent: 08/21/13 09:12 PM > On Wed, Aug 21, 2013 at 08:14:46PM +0200, Tibor Billes wrote: > > > From: Paul E. McKenney Sent: 08/20/13 11:43 PM > > > On Tue, Aug 20, 2013 at 10:52:26PM +0200, Tibor Billes wrote: > > > > > From: Paul E. McKenney Sent: 08/20/13 04:53 PM > > > > > On Tue, Aug 20, 2013 at 08:01:28AM +0200, Tibor Billes wrote: > > > > > > Hi, > > > > > > > > > > > > I was using the 3.9.7 stable release and tried to upgrade to the 3.10.x series. > > > > > > The 3.10.x series was showing unusually high (>75%) system CPU usage in some > > > > > > situations, making things really slow. The latest stable I tried is 3.10.7. > > > > > > I also tried 3.11-rc5, they both show this behaviour. This behaviour doesn't > > > > > > show up when the system is idling, only when doing some CPU intensive work, > > > > > > like compiling with multiple threads. Compiling with only one thread seems not > > > > > > to trigger this behaviour. > > > > > > > > > > > > To be more precise I did a `perf record -a` while compiling a large C++ program > > > > > > with scons using 4 threads, the result is appended at the end of this email. > > > > > > > > > > New one on me! You are running a mainstream system (x86_64), so I am > > > > > surprised no one else noticed. > > > > > > > > > > Could you please send along your .config file? > > > > > > > > Here it is > > > > > > Interesting. I don't see RCU stuff all that high on the list, but > > > the items I do see lead me to suspect RCU_FAST_NO_HZ, which has some > > > relevance to the otherwise inexplicable group of commits you located > > > with your bisection. Could you please rerun with CONFIG_RCU_FAST_NO_HZ=n? > > > > > > If that helps, there are some things I could try. > > > > It did help. I didn't notice anything unusual when running with CONFIG_RCU_FAST_NO_HZ=n. > > Interesting. Thank you for trying this -- and we at least have a > short-term workaround for this problem. I will put a patch together > for further investigation. I don't specifically need this config option so I'm fine without it in the long term, but I guess it's not supposed to behave like that. > In the meantime, could you please tell me how you were measuring > performance for your kernel builds? Wall-clock time required to complete > one build? Number of builds completed per unit time? Something else? Actually, I wasn't all this sophisticated. I have a system monitor applet on my top panel (using MATE, Linux Mint), four little graphs, one of which shows CPU usage. Different colors indicate different kind of CPU usage. Blue shows user space usage, red shows system usage, and two more for nice and iowait. During a normal compile it's almost completely filled with blue user space CPU usage, only the top few pixels show some iowait and system usage. With CONFIG_RCU_FAST_NO_HZ set, about 3/4 of the graph was red system CPU usage, the rest was blue. So I just looked for a pile of red on my graphs when I tested different kernel builds. But also compile speed was horrible I couldn't wait for the build to finish. Even the UI got unresponsive. Now I did some measuring. In the normal case a compile finished in 36 seconds, compiled 315 object files. Here are some output lines from dstat -tasm --vm during compile: ----system---- ----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system-- ----swap--- ------memory-usage----- -----virtual-memory---- time |usr sys idl wai hiq siq| read writ| recv send| in out | int csw | used free| used buff cach free|majpf minpf alloc free 21-08 21:48:05| 91 8 2 0 0 0| 0 5852k| 0 0 | 0 0 |1413 1772 | 0 7934M| 581M 58.0M 602M 6553M| 0 71k 46k 54k 21-08 21:48:06| 93 6 1 0 0 0| 0 2064k| 137B 131B| 0 0 |1356 1650 | 0 7934M| 649M 58.0M 604M 6483M| 0 72k 47k 28k 21-08 21:48:07| 86 11 4 0 0 0| 0 5872k| 0 0 | 0 0 |2000 2991 | 0 7934M| 577M 58.0M 627M 6531M| 0 99k 67k 79k 21-08 21:48:08| 87 9 3 0 0 0| 0 2840k| 0 0 | 0 0 |2558 4164 | 0 7934M| 597M 58.0M 632M 6507M| 0 96k 57k 51k 21-08 21:48:09| 93 7 1 0 0 0| 0 3032k| 0 0 | 0 0 |1329 1512 | 0 7934M| 641M 58.0M 626M 6469M| 0 61k 48k 39k 21-08 21:48:10| 93 6 0 0 0 0| 0 4984k| 0 0 | 0 0 |1160 1146 | 0 7934M| 572M 58.0M 628M 6536M| 0 50k 40k 57k 21-08 21:48:11| 86 9 6 0 0 0| 0 2520k| 0 0 | 0 0 |2947 4760 | 0 7934M| 605M 58.0M 631M 6500M| 0 103k 55k 45k 21-08 21:48:12| 90 8 2 0 0 0| 0 2840k| 0 0 | 0 0 |2674 4179 | 0 7934M| 671M 58.0M 635M 6431M| 0 84k 59k 42k 21-08 21:48:13| 90 9 1 0 0 0| 0 4656k| 0 0 | 0 0 |1223 1410 | 0 7934M| 643M 58.0M 638M 6455M| 0 90k 62k 68k 21-08 21:48:14| 91 8 1 0 0 0| 0 3572k| 0 0 | 0 0 |1432 1828 | 0 7934M| 647M 58.0M 641M 6447M| 0 81k 59k 57k 21-08 21:48:15| 91 8 1 0 0 0| 0 5116k| 116B 0 | 0 0 |1194 1295 | 0 7934M| 605M 58.0M 644M 6487M| 0 69k 54k 64k 21-08 21:48:16| 87 10 3 0 0 0| 0 5140k| 0 0 | 0 0 |1761 2586 | 0 7934M| 584M 58.0M 650M 6502M| 0 105k 64k 68k The abnormal case compiled only 182 object file in 6 and a half minutes, then I stopped it. The same dstat output for this case: ----system---- ----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system-- ----swap--- ------memory-usage----- -----virtual-memory---- time |usr sys idl wai hiq siq| read writ| recv send| in out | int csw | used free| used buff cach free|majpf minpf alloc free 21-08 22:10:49| 27 62 0 0 11 0| 0 0 | 210B 0 | 0 0 |1414 3137k| 0 7934M| 531M 57.6M 595M 6611M| 0 1628 1250 322 21-08 22:10:50| 25 60 4 0 11 0| 0 88k| 126B 0 | 0 0 |1337 3110k| 0 7934M| 531M 57.6M 595M 6611M| 0 91 128 115 21-08 22:10:51| 26 63 0 0 11 0| 0 184k| 294B 0 | 0 0 |1411 3147k| 0 7934M| 531M 57.6M 595M 6611M| 0 1485 814 815 21-08 22:10:52| 26 63 0 0 11 0| 0 0 | 437B 239B| 0 0 |1355 3160k| 0 7934M| 531M 57.6M 595M 6611M| 0 24 94 97 21-08 22:10:53| 26 63 0 0 11 0| 0 0 | 168B 0 | 0 0 |1397 3155k| 0 7934M| 531M 57.6M 595M 6611M| 0 479 285 273 21-08 22:10:54| 26 63 0 0 11 0| 0 4096B| 396B 324B| 0 0 |1346 3154k| 0 7934M| 531M 57.6M 595M 6611M| 0 27 145 145 21-08 22:10:55| 26 63 0 0 11 0| 0 60k| 0 0 | 0 0 |1353 3148k| 0 7934M| 531M 57.6M 595M 6610M| 0 93 117 36 21-08 22:10:56| 26 63 0 0 11 0| 0 0 | 0 0 | 0 0 |1341 3172k| 0 7934M| 531M 57.6M 595M 6610M| 0 158 87 74 21-08 22:10:57| 26 62 1 0 11 0| 0 0 | 42B 60B| 0 0 |1332 3162k| 0 7934M| 531M 57.6M 595M 6610M| 0 56 82 78 21-08 22:10:58| 26 63 0 0 11 0| 0 0 | 0 0 | 0 0 |1334 3178k| 0 7934M| 531M 57.6M 595M 6610M| 0 26 56 56 21-08 22:10:59| 26 63 0 0 11 0| 0 0 | 0 0 | 0 0 |1336 3179k| 0 7934M| 531M 57.6M 595M 6610M| 0 3 33 32 21-08 22:11:00| 26 63 0 0 11 0| 0 24k| 90B 108B| 0 0 |1347 3172k| 0 7934M| 531M 57.6M 595M 6610M| 0 41 73 71 I have four logical cores so 25% makes up 1 core. I don't know if the ~26% user CPU usage has anthing to do with this fact or just coincidence. The rest is ~63% system and ~11% hardware interrupt. Do these support what you suspect? Tibor -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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