Re: [PATCH 0/3] Shrinkers and proportional reclaim

[Konstantin Khlebnikov]

On Wed, May 28, 2014 at 1:17 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> On Tue, 27 May 2014, Dave Chinner wrote:
>> On Mon, May 26, 2014 at 02:44:29PM -0700, Hugh Dickins wrote:
>> >
>> > [PATCH 4/3] fs/superblock: Avoid counting without __GFP_FS
>> >
>> > Don't waste time counting objects in super_cache_count() if no __GFP_FS:
>> > super_cache_scan() would only back out with SHRINK_STOP in that case.
>> >
>> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
>>
>> While you might think that's a good thing, it's not.  The act of
>> shrinking is kept separate from the accounting of how much shrinking
>> needs to take place.  The amount of work the shrinker can't do due
>> to the reclaim context is deferred until the shrinker is called in a
>> context where it can do work (eg. kswapd)
>>
>> Hence not accounting for work that can't be done immediately will
>> adversely impact the balance of the system under memory intensive
>> filesystem workloads. In these worklaods, almost all allocations are
>> done in the GFP_NOFS or GFP_NOIO contexts so not deferring the work
>> will will effectively stop superblock cache reclaim entirely....
>
> Thanks for filling me in on that.  At first I misunderstood you,
> and went off looking in the wrong direction.  Now I see what you're
> referring to: the quantity that shrink_slab_node() accumulates in
> and withdraws from shrinker->nr_deferred[nid].

Maybe shrinker could accumulate fraction nr_pages_scanned / lru_pages
instead of exact amount of required work? Count of shrinkable objects
might be calculated later, when shrinker is called from a suitable context and
can actualy do something.
--
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/