Re: Bad SSD performance with recent kernels

[Shaohua Li]

On Mon, 2012-01-30 at 08:13 +0100, Herbert Poetzl wrote:
> On Mon, Jan 30, 2012 at 11:17:38AM +0800, Shaohua Li wrote:
> > 2012/1/30 Wu Fengguang <wfg@xxxxxxxxxxxxxxx>:
> >> On Sun, Jan 29, 2012 at 02:13:51PM +0100, Eric Dumazet wrote:
> >>> Le dimanche 29 janvier 2012 Ã 19:16 +0800, Wu Fengguang a Ãcrit :
> 
> 
> >>>> Note that as long as buffered read(2) is used, it makes almost no
> >>>> difference (well, at least for now) to do "dd bs=128k" or "dd bs=2MB":
> >>>> the 128kb readahead size will be used underneath to submit read IO.
> 
> 
> >>> Hmm...
> 
> >>> # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
> >>> 32768+0 enregistrements lus
> >>> 32768+0 enregistrements Ãcrits
> >>> 4294967296 octets (4,3 GB) copiÃs, 20,7718 s, 207 MB/s
> 
> 
> >>> # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
> >>> 2048+0 enregistrements lus
> >>> 2048+0 enregistrements Ãcrits
> >>> 4294967296 octets (4,3 GB) copiÃs, 27,7824 s, 155 MB/s
> 
> >> Interesting. Here are my test results:
> 
> >> root@lkp-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
> >> 32768+0 records in
> >> 32768+0 records out
> >> 4294967296 bytes (4.3 GB) copied, 19.0121 s, 226 MB/s
> >> root@lkp-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
> >> 2048+0 records in
> >> 2048+0 records out
> >> 4294967296 bytes (4.3 GB) copied, 19.0214 s, 226 MB/s
> 
> >> Maybe the /dev/sda performance bug on your machine is sensitive to timing?
> > I got similar result:
> > 128k: 224M/s
> > 1M: 182M/s
> 
> > 1M block size is slow, I guess it's CPU related.
> 
> > And as for the big regression with newer kernel than 2.6.38,
> > please check if idle=poll helps. CPU idle dramatically impacts
> > disk performance and even latest cpuidle governor doesn't help
> > for some CPUs.
> 
> here are the tests with idle=poll and after switching to 128k
> (instead of 1M) blocksize (same amount of data transferred)
> 
> kernel    ------------ read /dev/sda -------------
>           --- noop ---  - deadline -  ---- cfs ---
>           [MB/s]  %CPU  [MB/s]  %CPU  [MB/s]  %CPU
> --------------------------------------------------
> 3.2.2      45.82   3.7   44.85   3.6   45.04   3.4
> 3.2.2i     45.59   2.3   51.78   2.6   46.03   2.2
> 3.2.2i128 250.24  20.9  252.68  21.3  250.00  21.6
> 
> kernel    -- write ---  ------------------read -----------------
>           --- noop ---  --- noop ---  - deadline -  ---- cfs ---
>           [MB/s]  %CPU  [MB/s]  %CPU  [MB/s]  %CPU  [MB/s]  %CPU
> ----------------------------------------------------------------
> 3.2.2     270.95  42.6  162.36   9.9  162.63   9.9  162.65  10.1
> 3.2.2i    269.10  41.4  170.82   6.6  171.20   6.6  170.91   6.7
> 3.2.2i128 270.38  67.7  162.35  10.2  163.01  10.3  162.34  10.7
What's 3.2.2i and 3.2.2i128? does idle=poll help?

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/