Re: [PATCH 01/29] iov_iter: Switch to using a table of operations

[David Howells]

Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:

> Please run performance tests.  I think the indirect calls could totally
> wreck things like high performance direct I/O, especially using io_uring
> on x86.

Here's an initial test using fio and null_blk.  I left null_blk in its default
configuration and used the following command line:

fio --ioengine=libaio --direct=1 --gtod_reduce=1 --name=readtest --filename=/dev/nullb0 --bs=4k --iodepth=128 --time_based --runtime=120 --readwrite=randread --iodepth_low=96 --iodepth_batch=16 --numjobs=4

I borrowed some of the parameters from an email I found online, so I'm not
sure if they're that useful.

I tried three different sets of patches: none, just the first (which adds the
jump table without getting rid of the conditional branches), and all of them.

I'm not sure which stats are of particular interest here, so I took the two
summary stats from the output of fio and also added together the "issued rwts:
total=a,b,c,d" from each test thread (only the first of which is non-zero).

The CPU is an Intel(R) Core(TM) i3-4170 CPU @ 3.70GHz, so 4 single-thread
cores, and 16G of RAM.  No virtualisation is involved.

Unpatched:

   READ: bw=4109MiB/s (4308MB/s), 1025MiB/s-1029MiB/s (1074MB/s-1079MB/s), io=482GiB (517GB), run=120001-120001msec
   READ: bw=4097MiB/s (4296MB/s), 1020MiB/s-1029MiB/s (1070MB/s-1079MB/s), io=480GiB (516GB), run=120001-120001msec
   READ: bw=4113MiB/s (4312MB/s), 1025MiB/s-1031MiB/s (1075MB/s-1082MB/s), io=482GiB (517GB), run=120001-120001msec
   READ: bw=4125MiB/s (4325MB/s), 1028MiB/s-1033MiB/s (1078MB/s-1084MB/s), io=483GiB (519GB), run=120001-120001msec

  nullb0: ios=126017326/0, merge=53/0, ticks=3538817/0, in_queue=3538817, util=100.00%
  nullb0: ios=125655193/0, merge=55/0, ticks=3548157/0, in_queue=3548157, util=100.00%
  nullb0: ios=126133014/0, merge=58/0, ticks=3545621/0, in_queue=3545621, util=100.00%
  nullb0: ios=126512562/0, merge=57/0, ticks=3531600/0, in_queue=3531600, util=100.00%

  sum issued rwts = 126224632
  sum issued rwts = 125861368
  sum issued rwts = 126340344
  sum issued rwts = 126718648

Just first patch:

   READ: bw=4106MiB/s (4306MB/s), 1023MiB/s-1030MiB/s (1073MB/s-1080MB/s), io=481GiB (517GB), run=120001-120001msec
   READ: bw=4126MiB/s (4327MB/s), 1029MiB/s-1034MiB/s (1079MB/s-1084MB/s), io=484GiB (519GB), run=120001-120001msec
   READ: bw=4109MiB/s (4308MB/s), 1025MiB/s-1029MiB/s (1075MB/s-1079MB/s), io=481GiB (517GB), run=120001-120001msec
   READ: bw=4097MiB/s (4296MB/s), 1023MiB/s-1025MiB/s (1073MB/s-1074MB/s), io=480GiB (516GB), run=120001-120001msec

  nullb0: ios=125939152/0, merge=62/0, ticks=3534917/0, in_queue=3534917, util=100.00%
  nullb0: ios=126554181/0, merge=61/0, ticks=3532067/0, in_queue=3532067, util=100.00%
  nullb0: ios=126012346/0, merge=54/0, ticks=3530504/0, in_queue=3530504, util=100.00%
  nullb0: ios=125653775/0, merge=54/0, ticks=3537438/0, in_queue=3537438, util=100.00%

  sum issued rwts = 126144952
  sum issued rwts = 126765368
  sum issued rwts = 126215928
  sum issued rwts = 125864120

All patches:
  nullb0: ios=10477062/0, merge=2/0, ticks=284992/0, in_queue=284992, util=95.87%
  nullb0: ios=10405246/0, merge=2/0, ticks=291886/0, in_queue=291886, util=99.82%
  nullb0: ios=10425583/0, merge=1/0, ticks=291699/0, in_queue=291699, util=99.22%
  nullb0: ios=10438845/0, merge=3/0, ticks=292445/0, in_queue=292445, util=99.31%

   READ: bw=4118MiB/s (4318MB/s), 1028MiB/s-1032MiB/s (1078MB/s-1082MB/s), io=483GiB (518GB), run=120001-120001msec
   READ: bw=4109MiB/s (4308MB/s), 1024MiB/s-1030MiB/s (1073MB/s-1080MB/s), io=481GiB (517GB), run=120001-120001msec
   READ: bw=4108MiB/s (4308MB/s), 1026MiB/s-1029MiB/s (1076MB/s-1079MB/s), io=481GiB (517GB), run=120001-120001msec
   READ: bw=4112MiB/s (4312MB/s), 1025MiB/s-1031MiB/s (1075MB/s-1081MB/s), io=482GiB (517GB), run=120001-120001msec

  nullb0: ios=126282410/0, merge=58/0, ticks=3557384/0, in_queue=3557384, util=100.00%
  nullb0: ios=126004837/0, merge=67/0, ticks=3565235/0, in_queue=3565235, util=100.00%
  nullb0: ios=125988876/0, merge=59/0, ticks=3563026/0, in_queue=3563026, util=100.00%
  nullb0: ios=126118279/0, merge=57/0, ticks=3566122/0, in_queue=3566122, util=100.00%

  sum issued rwts = 126494904
  sum issued rwts = 126214200
  sum issued rwts = 126198200
  sum issued rwts = 126328312


David