[Qemu-devel] Re: Guest latency issues due to bdrv_check_byte_request

[Jan Kiszka]

Stefan Hajnoczi wrote:
> Thanks Christoph.
> 
> Cached getlength with pread/pwrite:
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  96.97    1.760111       11893       148         4 futex
>   1.61    0.029209           1     46891      2217 select
>   0.28    0.005047           0     64609           timer_gettime
>   0.22    0.004059           0     42745      2578 rt_sigreturn
>   0.22    0.003911           0     46261           timer_settime
>   0.18    0.003280        1093         3           shmdt
>   0.17    0.003095           0     23859           pread  <---
>   0.17    0.003061           0     42800           write
>   0.16    0.002916           0     47759      5151 read
>   0.02    0.000285           0       645           writev
> [...]
>   0.00    0.000000           0        13           lseek
> 
> Note that this is a Tiny Core Linux boot from disk and shutdown; not
> very I/O intensive since it only loads a kernel and ~10 MB initramfs
> without touching the disk much after kernel load.

Nice. Will give this a try tomorrow with "a bit" more load.

We already played with a hack to completely remove the checks from AIO
requests, thus avoiding lseek this way - effect as desired, but fragile
of course.

Jan

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