Re: [PATCH 08/13] fs: add read support for RWF_UNCACHED

[Stefan Metzmacher]

Hi Jens,

> If the same test case is run with RWF_UNCACHED set for the buffered read,
> the output looks as follows:
>
> Reading bs 65536, uncached 0
>    1s: 153144MB/sec
>    2s: 156760MB/sec
>    3s: 158110MB/sec
>    4s: 158009MB/sec
>    5s: 158043MB/sec
>    6s: 157638MB/sec
>    7s: 157999MB/sec
>    8s: 158024MB/sec
>    9s: 157764MB/sec
>   10s: 157477MB/sec
>   11s: 157417MB/sec
>   12s: 157455MB/sec
>   13s: 157233MB/sec
>   14s: 156692MB/sec
>
> which is just chugging along at ~155GB/sec of read performance. Looking
> at top, we see:
>
>   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
> 7961 root      20   0  267004      0      0 S  3180   0.0   5:37.95 uncached
> 8024 axboe     20   0   14292   4096      0 R   1.0   0.0   0:00.13 top
>
> where just the test app is using CPU, no reclaim is taking place outside
> of the main thread. Not only is performance 65% better, it's also using
> half the CPU to do it.

Do you have numbers of similar code using O_DIRECT just to
see the impact of the memcpy from the page cache to the userspace
buffer...

Thanks!
metze