Re: [PATCH 2/2 v2] writeback: Add writeback stats for pages written

[Curt Wohlgemuth <curtw@google.com>]

Hi Fengguang:

Thanks for looking at this.

On Mon, Aug 15, 2011 at 6:48 AM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> Curt,
>
> Some thoughts about the interface..before dipping into the code.
>
> On Sat, Aug 13, 2011 at 06:47:25AM +0800, Curt Wohlgemuth wrote:
>> Add a new file, /proc/writeback/stats, which displays
>
> That's creating a new top directory in /proc. Do you have plans for
> adding more files under it?

Good question.  We have several files under /proc/writeback in our
kernels that we created at various times, some of which are probably
no longer useful, but others seem to be.  For example:
  - congestion: prints # of calls, # of jiffies slept in
congestion_wait() / io_schedule_timeout() from various call points
  - threshold_dirty : prints the current global FG threshold
  - threshold_bg : prints the current global BG threshold
  - pages_cleaned : prints the # pages sent to writeback -- same as
'nr_written' in /proc/vmstat (ours was earlier :-( )
  - pages_dirtied (same as nr_dirtied in /proc/vmstat)
  - prop_vm_XXX : print shift/events from vm_completions and vm_dirties

I'm not sure right now if global FG/BG thresholds appear anywhere in a
3.1 kernel; if so, the two threshold files above are superfluous.  So
are the pages_cleaned/dirtied.  The prop_vm files have not proven
useful to me.  I think the congestion file has a lot of value,
especially in an IO-less throttling world...

>
>> machine global data for how many pages were cleaned for
>> which reasons.  It also displays some additional counts for
>> various writeback events.
>>
>> These data are also available for each BDI, in
>> /sys/block/<device>/bdi/writeback_stats .
>
>> Sample output:
>>
>>    page: balance_dirty_pages           2561544
>>    page: background_writeout              5153
>>    page: try_to_free_pages                   0
>>    page: sync                                0
>>    page: kupdate                        102723
>>    page: fdatawrite                    1228779
>>    page: laptop_periodic                     0
>>    page: free_more_memory                    0
>>    page: fs_free_space                       0
>>    periodic writeback                      377
>>    single inode wait                         0
>>    writeback_wb wait                         1
>
> That's already useful data, and could be further extended (in
> future patches) to answer questions like "what's the writeback
> efficiency in terms of effective chunk size?"
>
> So in future there could be lines like
>
>    pages: balance_dirty_pages           2561544
>    chunks: balance_dirty_pages          XXXXXXX
>    works: balance_dirty_pages           XXXXXXX
>
> or even derived lines like
>
>    pages_per_chunk: balance_dirty_pages         XXXXXXX
>    pages_per_work: balance_dirty_pages          XXXXXXX
>
> Another question is, how can the display format be script friendly?
> The current form looks not easily parse-able at least for "cut"..

I suppose you mean because of the variable number of tokens.  Yeah,
this can be hard.  Of course, I always just use "awk '{print $NF}'"
and it works for me :-) .  But I'd be happy to change these to use a
consistent # of args.

Thanks,
Curt


> Thanks,
> Fengguang
>

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