alinux: jbd2: add proc entry to control whether doing buffer copy-out
When jbd2 tries to get write access to one buffer, and if this buffer is under writeback with BH_Shadow flag, jbd2 will wait until this buffer has been written to disk, but sometimes the time taken to wait may be much long, especially disk capacity is almost full. Here add a proc entry "force-copy", if its value is not zero, jbd2 will always do meta buffer copy-cout, then we can eliminate the unnecessary wating time here, and reduce long tail latency for buffered-write. I construct such test case below: $cat offline.fio ; fio-rand-RW.job for fiotest [global] name=fio-rand-RW filename=fio-rand-RW rw=randrw rwmixread=60 rwmixwrite=40 bs=4K direct=0 numjobs=4 time_based=1 runtime=900 [file1] size=60G ioengine=sync iodepth=16 $cat online.fio ; fio-seq-write.job for fiotest [global] name=fio-seq-write filename=fio-seq-write rw=write bs=256K direct=0 numjobs=1 time_based=1 runtime=60 [file1] rate=50m size=10G ioengine=sync iodepth=16 With this patch: $cat /proc/fs/jbd2/sda5-8/force_copy 0 online fio almost always get such long tail latency: Jobs: 1 (f=1), 0B/s-0B/s: [W(1)][100.0%][w=50.0MiB/s][w=200 IOPS][eta 00m:00s] file1: (groupid=0, jobs=1): err= 0: pid=17855: Thu Nov 15 09:45:57 2018 write: IOPS=200, BW=50.0MiB/s (52.4MB/s)(3000MiB/60001msec) clat (usec): min=135, max=4086.6k, avg=867.21, stdev=50338.22 lat (usec): min=139, max=4086.6k, avg=871.16, stdev=50338.22 clat percentiles (usec): | 1.00th=[ 141], 5.00th=[ 143], 10.00th=[ 145], | 20.00th=[ 147], 30.00th=[ 147], 40.00th=[ 149], | 50.00th=[ 149], 60.00th=[ 151], 70.00th=[ 153], | 80.00th=[ 155], 90.00th=[ 159], 95.00th=[ 163], | 99.00th=[ 255], 99.50th=[ 273], 99.90th=[ 429], | 99.95th=[ 441], 99.99th=[3640656] $cat /proc/fs/jbd2/sda5-8/force_copy 1 online fio latency is much better. Jobs: 1 (f=1), 0B/s-0B/s: [W(1)][100.0%][w=50.0MiB/s][w=200 IOPS][eta 00m:00s] file1: (groupid=0, jobs=1): err= 0: pid=8084: Thu Nov 15 09:31:15 2018 write: IOPS=200, BW=50.0MiB/s (52.4MB/s)(3000MiB/60001msec) clat (usec): min=137, max=545, avg=151.35, stdev=16.22 lat (usec): min=140, max=548, avg=155.31, stdev=16.65 clat percentiles (usec): | 1.00th=[ 143], 5.00th=[ 145], 10.00th=[ 145], 20.00th=[ 147], | 30.00th=[ 147], 40.00th=[ 147], 50.00th=[ 149], 60.00th=[ 149], | 70.00th=[ 151], 80.00th=[ 155], 90.00th=[ 157], 95.00th=[ 161], | 99.00th=[ 239], 99.50th=[ 269], 99.90th=[ 420], 99.95th=[ 429], | 99.99th=[ 537] As to the cost: because we'll always need to copy meta buffer, will consume minor cpu time and some memory(at most 32MB for 128MB journal size). Signed-off-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com> Reviewed-by: NLiu Bo <bo.liu@linux.alibaba.com> Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com> Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>
Showing
想要评论请 注册 或 登录