Re: Page Cache writeback too slow, SSD/noop scheduler/ext2

[Andrew Morton]

On Fri, 20 Mar 2009 19:26:06 +0100 Jos Houtman <jos@xxxxxxxx> wrote:

> Hi,
> 
> We have hit a problem where the page-cache writeback algorithm is not
> keeping up.
> When memory gets low this will result in very irregular performance drops.
> 
> Our setup is as follows:
> 30 x Quad core machine with 64GB ram.
> These are single purpose machines running MySQL.
> Kernel version: 2.6.28.7
> A dedicated SSD drive for the ext2 database partition
> Noop scheduler for the ssd drive.
> 
> 
> The current hypothesis is as follows:
> The wk_update function does not write enough dirty pages, which allows the
> number of dirty pages to grow to the dirty_background limit.
> When memory is low,  __background_writeout() comes around and __forcefully__
> writes dirty pages to disk.
> This forced write fills the disk queue and starves read calls that MySQL is
> trying to do: basically killing performance  for a few seconds.
> This pattern repeats as soon as the cleared memory is filled again.
> 
> Decreasing the dirty_writeback_centisecs to 100 doesn__t help
> 
> I don__t know why this is, but I did some preliminary tracing using systemtap
> and it seems that the majority of times wk_update calls decides to do
> nothing.
> 
> Doubling /sys/block/sdb/queue/nr_requests  to 256, seems to help abit:  the
> nr_dirty pages is increasing more slowly.
> But I am unsure of side-effects and am afraid of increasing the starvation
> problem for mysql.
> 
> 
> I__am very much willing to work on this issue and see it fixed, but would
> like to tap into the knowledge of people here.
> So: 
> * Have more people seen this or simular issues?
> * Is the hypothesis above a viable one?
> * Suggestions/pointers for further research and statistics I should measure
> to improve the understanding of this problem.
> 

I don't think that noop-iosched tries to do anything to prevent
writes-starve-reads.  Do you get better behaviour from any of the other IO
schedulers?


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