On Mon, Sep 20, 2021 at 12:42:44PM +0100, Matthew Wilcox wrote: > On Mon, Sep 20, 2021 at 09:54:31AM +0100, Mel Gorman wrote: > > This has been lightly tested only and the testing was useless as the > > relevant code was not executed. The workload configurations I had that > > used to trigger these corner cases no longer work (yey?) and I'll need > > to implement a new synthetic workload. If someone is aware of a realistic > > workload that forces reclaim activity to the point where reclaim stalls > > then kindly share the details. > > The stereeotypical "stalling on I/O" problem is to plug in one of the > crap USB drives you were given at a trade show and simply > dd if=/dev/zero of=/dev/sdb > sync > > You can also set up qemu to have extremely slow I/O performance: > https://serverfault.com/questions/675704/extremely-slow-qemu-storage-performance-with-qcow2-images > Ok, I managed to get something working and nothing blew up. The workload was similar to what I described except the dirty file data is related to dirty_ratio, the memory hogs no longer sleep and I disabled the parallel readers. There is still a configuration with the parallel readers but I won't have the results till tomorrow. Surprising no one, vanilla kernel throttling barely works. 1 writeback_wait_iff_congested: usec_delayed=4000 3 writeback_congestion_wait: usec_delayed=108000 196 writeback_congestion_wait: usec_delayed=104000 16697 writeback_wait_iff_congested: usec_delayed=0 too_many_isolated it not tracked at all so we don't know what that looks like but kswapd "blocking" on dirty pages at the tail basically never stalls. The few congestion_wait's that did happen stalled for the full duration as the bdi is not tracking congestion at all. With the series, the breakdown of reasons to stall were 5703 reason=VMSCAN_THROTTLE_WRITEBACK 29644 reason=VMSCAN_THROTTLE_NOPROGRESS 1979999 reason=VMSCAN_THROTTLE_ISOLATED kswapd stalls were rare but they did happen and surprise surprise, it was dirty pages 914 reason=VMSCAN_THROTTLE_WRITEBACK All of them stalled for the full timeout so there might be a bug in patch 1 because that sounds suspicious. As "too many pages isolated" was the top reason, the frequency of each stall time is as follows 1 usect_delayed=164000 1 usect_delayed=192000 1 usect_delayed=200000 1 usect_delayed=208000 1 usect_delayed=220000 1 usect_delayed=244000 1 usect_delayed=308000 1 usect_delayed=312000 1 usect_delayed=316000 1 usect_delayed=332000 1 usect_delayed=588000 1 usect_delayed=620000 1 usect_delayed=836000 3 usect_delayed=116000 4 usect_delayed=124000 4 usect_delayed=128000 6 usect_delayed=120000 9 usect_delayed=112000 11 usect_delayed=100000 13 usect_delayed=48000 13 usect_delayed=96000 14 usect_delayed=40000 15 usect_delayed=88000 15 usect_delayed=92000 16 usect_delayed=80000 18 usect_delayed=68000 19 usect_delayed=76000 22 usect_delayed=84000 23 usect_delayed=108000 23 usect_delayed=60000 25 usect_delayed=44000 25 usect_delayed=52000 29 usect_delayed=36000 30 usect_delayed=56000 30 usect_delayed=64000 33 usect_delayed=72000 57 usect_delayed=32000 91 usect_delayed=20000 107 usect_delayed=24000 125 usect_delayed=28000 131 usect_delayed=16000 180 usect_delayed=12000 186 usect_delayed=8000 1379 usect_delayed=104000 16493 usect_delayed=4000 1960837 usect_delayed=0 In other words, the vast majority of stalls were for 0 time and the task was immediately woken again. The next most common stall time was 1 tick but a sizable number reach the full timeout. Everything else is somewhere in between so the event trigger appears to be ok. I don't know how the application itself performed as I still have to write the analysis script and assuming I can look at this tomorrow, I'll probably start with why VMSCAN_THROTTLE_WRITEBACK always stalled for the full timeout. -- Mel Gorman SUSE Labs
