Re: Bad SSD performance with recent kernels

[Wu Fengguang]

> - IO size is 256KB (which is not a problem in itself)
> 
> - The dispatch/complete pattern is
> 
>         submit IO for range 1
>         complete IO for range 1
>         <dd busy, disk idle>
>         submit IO for range 2
>         complete IO for range 2
> 
>   So we have periods that no IO is in flight at all, which leads to
>   under-utilized disk (which should show up in iostat as <100% disk util)
> 
> # grep '[DC]' blktrace
> 
>   8,16   1   770606     9.990039807  4580  D   R 4379392 + 512 (   14826) [dd]
>   8,16   1   770607     9.991083069     0  C   R 4379392 + 512 ( 1043262) [0]
> 
>   8,16   1   770739     9.991647434  4580  D   R 4379904 + 512 (   14433) [dd]
>   8,16   1   770740     9.992693317     0  C   R 4379904 + 512 ( 1045883) [0]
> 
>   8,16   1   770872     9.993256451  4580  D   R 4380416 + 512 (   14539) [dd]
>   8,16   1   770873     9.994299156     0  C   R 4380416 + 512 ( 1042705) [0]
> 
>   8,16   1   771005     9.994863680  4580  D   R 4380928 + 512 (   14344) [dd]
>   8,16   1   771006     9.995909291     0  C   R 4380928 + 512 ( 1045611) [0]
> 
>   8,16   1   771138     9.996470460  4580  D   R 4381440 + 512 (   14043) [dd]
>   8,16   1   771139     9.997514205     0  C   R 4381440 + 512 ( 1043745) [0]
> 
>   8,16   1   771271     9.998077269  4580  D   R 4381952 + 512 (   14928) [dd]
>   8,16   1   771272     9.999120396     0  C   R 4381952 + 512 ( 1043127) [0]

Some better pattern for comparison:

  8,0    1    42940     0.990199812  4084  D   R 1432808 + 256 [dd]
  8,0    1    42941     0.990858326     0  C   R 1432552 + 256 [0]
  8,0    1    43009     0.991192107  4084  D   R 1433064 + 256 [dd]
  8,0    3    14691     0.991853319     0  C   R 1432808 + 256 [0]
  8,0    3    14759     0.992189451  4084  D   R 1433320 + 256 [dd]
  8,0    1    43010     0.994473159     0  C   R 1433064 + 256 [0]

Where the life time for requests 1432808, 1432552, 1433064, ...
are *interleaved* so that the disk always has something to do.

Thanks,
Fengguang
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