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Date: Mon, 21 Jan 2013 05:38:48 +0100 From: Mike Galbraith <bitbucket@...ine.de> To: Michael Wang <wangyun@...ux.vnet.ibm.com> Cc: linux-kernel@...r.kernel.org, mingo@...hat.com, peterz@...radead.org, mingo@...nel.org, a.p.zijlstra@...llo.nl Subject: Re: [RFC PATCH 0/2] sched: simplify the select_task_rq_fair() On Mon, 2013-01-21 at 10:50 +0800, Michael Wang wrote: > On 01/20/2013 12:09 PM, Mike Galbraith wrote: > > On Thu, 2013-01-17 at 13:55 +0800, Michael Wang wrote: > >> Hi, Mike > >> > >> I've send out the v2, which I suppose it will fix the below BUG and > >> perform better, please do let me know if it still cause issues on your > >> arm7 machine. > > > > s/arm7/aim7 > > > > Someone swiped half of CPUs/ram, so the box is now 2 10 core nodes vs 4. > > > > stock scheduler knobs > > > > 3.8-wang-v2 avg 3.8-virgin avg vs wang > > Tasks jobs/min > > 1 436.29 435.66 435.97 435.97 437.86 441.69 440.09 439.88 1.008 > > 5 2361.65 2356.14 2350.66 2356.15 2416.27 2563.45 2374.61 2451.44 1.040 > > 10 4767.90 4764.15 4779.18 4770.41 4946.94 4832.54 4828.69 4869.39 1.020 > > 20 9672.79 9703.76 9380.80 9585.78 9634.34 9672.79 9727.13 9678.08 1.009 > > 40 19162.06 19207.61 19299.36 19223.01 19268.68 19192.40 19056.60 19172.56 .997 > > 80 37610.55 37465.22 37465.22 37513.66 37263.64 37120.98 37465.22 37283.28 .993 > > 160 69306.65 69655.17 69257.14 69406.32 69257.14 69306.65 69257.14 69273.64 .998 > > 320 111512.36 109066.37 111256.45 110611.72 108395.75 107913.19 108335.20 108214.71 .978 > > 640 142850.83 148483.92 150851.81 147395.52 151974.92 151263.65 151322.67 151520.41 1.027 > > 1280 52788.89 52706.39 67280.77 57592.01 189931.44 189745.60 189792.02 189823.02 3.295 > > 2560 75403.91 52905.91 45196.21 57835.34 217368.64 217582.05 217551.54 217500.74 3.760 > > > > sched_latency_ns = 24ms > > sched_min_granularity_ns = 8ms > > sched_wakeup_granularity_ns = 10ms > > > > 3.8-wang-v2 avg 3.8-virgin avg vs wang > > Tasks jobs/min > > 1 436.29 436.60 434.72 435.87 434.41 439.77 438.81 437.66 1.004 > > 5 2382.08 2393.36 2451.46 2408.96 2451.46 2453.44 2425.94 2443.61 1.014 > > 10 5029.05 4887.10 5045.80 4987.31 4844.12 4828.69 4844.12 4838.97 .970 > > 20 9869.71 9734.94 9758.45 9787.70 9513.34 9611.42 9565.90 9563.55 .977 > > 40 19146.92 19146.92 19192.40 19162.08 18617.51 18603.22 18517.95 18579.56 .969 > > 80 37177.91 37378.57 37292.31 37282.93 36451.13 36179.10 36233.18 36287.80 .973 > > 160 70260.87 69109.05 69207.71 69525.87 68281.69 68522.97 68912.58 68572.41 .986 > > 320 114745.56 113869.64 114474.62 114363.27 114137.73 114137.73 114137.73 114137.73 .998 > > 640 164338.98 164338.98 164618.00 164431.98 164130.34 164130.34 164130.34 164130.34 .998 > > 1280 209473.40 209134.54 209473.40 209360.44 210040.62 210040.62 210097.51 210059.58 1.003 > > 2560 242703.38 242627.46 242779.34 242703.39 244001.26 243847.85 243732.91 243860.67 1.004 > > > > As you can see, the load collapsed at the high load end with stock > > scheduler knobs (desktop latency). With knobs set to scale, the delta > > disappeared. > > Thanks for the testing, Mike, please allow me to ask few questions. > > What are those tasks actually doing? what's the workload? It's the canned aim7 compute load, mixed bag load weighted toward compute. Below is the workfile, should give you an idea. # @(#) workfile.compute:1.3 1/22/96 00:00:00 # Compute Server Mix FILESIZE: 100K POOLSIZE: 250M 50 add_double 30 add_int 30 add_long 10 array_rtns 10 disk_cp 30 disk_rd 10 disk_src 20 disk_wrt 40 div_double 30 div_int 50 matrix_rtns 40 mem_rtns_1 40 mem_rtns_2 50 mul_double 30 mul_int 30 mul_long 40 new_raph 40 num_rtns_1 50 page_test 40 series_1 10 shared_memory 30 sieve 20 stream_pipe 30 string_rtns 40 trig_rtns 20 udp_test > And I'm confusing about how those new parameter value was figured out > and how could them help solve the possible issue? Oh, that's easy. I set sched_min_granularity_ns such that last_buddy kicks in when a third task arrives on a runqueue, and set sched_wakeup_granularity_ns near minimum that still allows wakeup preemption to occur. Combined effect is reduced over-scheduling. > Do you have any idea about which part in this patch set may cause the issue? Nope, I'm as puzzled by that as you are. When the box had 40 cores, both virgin and patched showed over-scheduling effects, but not like this. With 20 cores, symptoms changed in a most puzzling way, and I don't see how you'd be directly responsible. > One change by designed is that, for old logical, if it's a wake up and > we found affine sd, the select func will never go into the balance path, > but the new logical will, in some cases, do you think this could be a > problem? Since it's the high load end, where looking for an idle core is most likely to be a waste of time, it makes sense that entering the balance path would hurt _some_, it isn't free.. except for twiddling preemption knobs making the collapse just go away. We're still going to enter that path if all cores are busy, no matter how I twiddle those knobs. > > I thought perhaps the bogus (shouldn't exist) CPU domain in mainline > > somehow contributes to the strange behavioral delta, but killing it made > > zero difference. All of these numbers for both trees were logged with > > the below applies, but as noted, it changed nothing. > > The patch set was supposed to do accelerate by reduce the cost of > select_task_rq(), so it should be harmless for all the conditions. Yeah, it should just save some cycles, but I like to eliminate known bugs when testing, just in case. -Mike -- 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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