On 9 October 2013 16:19, Michal Suchanek <hramrach@xxxxxxxxx> wrote:
> Hello,
>
> On 19 September 2013 12:13, Jan Kara <jack@xxxxxxx> wrote:
>> On Wed 18-09-13 16:56:08, Michal Suchanek wrote:
>>> On 17 September 2013 23:13, Jan Kara <jack@xxxxxxx> wrote:
>>> > Hello,
>>>
>>> The default for dirty_ratio/dirty_background_ratio is 60/40. Setting
>> Ah, that's not upstream default. Upstream has 20/10. In SLES we use 40/10
>> to better accomodate some workloads but 60/40 on 8 GB machines with
>> SATA drive really seems too much. That is going to give memory management a
>> headache.
>>
>> The problem is that a good SATA drive can do ~100 MB/s if we are
>> lucky and IO is sequential. Thus if you have 5 GB of dirty data to write,
>> it takes 50s at best to write it, with more random IO to image file it can
>> well take several minutes to write. That may cause some increased latency
>> when memory reclaim waits for writeback to clean some pages.
>>
>>> these to 5/2 gives about the same result as running the script that
>>> syncs every 5s. Setting to 30/10 gives larger data chunks and
>>> intermittent lockup before every chunk is written.
>>>
>>> It is quite possible to set kernel parameters that kill the kernel but
>>>
>>> 1) this is the default
>> Not upstream one so you should raise this with Debian I guess. 60/40
>> looks way out of reasonable range for todays machines.
>>
>>> 2) the parameter is set in units that do not prevent the issue in
>>> general (% RAM vs #blocks)
>> You can set the number of bytes instead of percentage -
>> /proc/sys/vm/dirty_bytes / dirty_background_bytes. It's just that proper
>> sizing depends on amount of memory, storage HW, workload. So it's more an
>> administrative task to set this tunable properly.
>>
>>> 3) WTH is the system doing? It's 4core 3GHz cpu so it can handle
>>> traversing a structure holding 800M data in the background. Something
>>> is seriously rotten somewhere.
>> Likely processes are waiting in direct reclaim for IO to finish. But that
>> is just guessing. Try running attached script (forgot to attach it to
>> previous email). You will need systemtap and kernel debuginfo installed.
>> The script doesn't work with all versions of systemtap (as it is sadly a
>> moving target) so if it fails, tell me your version of systemtap and I'll
>> update the script accordingly.
>
> This was fixed for me by the patch posted earlier by Hillf Danton so I
> guess this answers what the system was (not) doing:
>
> --- a/mm/vmscan.c Wed Sep 18 08:44:08 2013
> +++ b/mm/vmscan.c Wed Sep 18 09:31:34 2013
> @@ -1543,8 +1543,11 @@ shrink_inactive_list(unsigned long nr_to
> * implies that pages are cycling through the LRU faster than
> * they are written so also forcibly stall.
> */
> - if (nr_unqueued_dirty == nr_taken || nr_immediate)
> + if (nr_unqueued_dirty == nr_taken || nr_immediate) {
> + if (current_is_kswapd())
> + wakeup_flusher_threads(0, WB_REASON_TRY_TO_FREE_PAGES);
> congestion_wait(BLK_RW_ASYNC, HZ/10);
> + }
> }
>
> /*
>
> Also 75485363 is hopefully addressing this issue in mainline.
>
Actually, this was in 3.11 already and it did make the behaviour a bit
better but was not enough.
So is something like the vmscan.c patch going to make it into the
mainline kernel?
Thanks
Michal
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