On 2/22/2017 4:13 PM, Michal Hocko wrote:
> On Tue 14-02-17 14:54:12, akpm@xxxxxxxxxxxxxxxxxxxx wrote:
>> From: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
>> Subject: mm: vmscan: do not pass reclaimed slab to vmpressure
>>
>> During global reclaim, the nr_reclaimed passed to vmpressure includes the
>> pages reclaimed from slab. But the corresponding scanned slab pages is
>> not passed. There is an impact to the vmpressure values because of this.
>> While moving from kernel version 3.18 to 4.4, a difference is seen in the
>> vmpressure values for the same workload resulting in a different behaviour
>> of the vmpressure consumer. One such case is of a vmpressure based
>> lowmemorykiller. It is observed that the vmpressure events are received
>> late and less in number resulting in tasks not being killed at the right
>> time. The following numbers show the impact on reclaim activity due to
>> the change in behaviour of lowmemorykiller on a 4GB device. The test
>> launches a number of apps in sequence and repeats it multiple times.
>>
>> v4.4 v3.18
>> pgpgin 163016456 145617236
>> pgpgout 4366220 4188004
>> workingset_refault 29857868 26781854
>> workingset_activate 6293946 5634625
>> pswpin 1327601 1133912
>> pswpout 3593842 3229602
>> pgalloc_dma 99520618 94402970
>> pgalloc_normal 104046854 98124798
>> pgfree 203772640 192600737
>> pgmajfault 2126962 1851836
>> pgsteal_kswapd_dma 19732899 18039462
>> pgsteal_kswapd_normal 19945336 17977706
>> pgsteal_direct_dma 206757 131376
>> pgsteal_direct_normal 236783 138247
>> pageoutrun 116622 108370
>> allocstall 7220 4684
>> compact_stall 931 856
>>
>> This is a regression introduced by commit 6b4f7799c6a5 ("mm: vmscan:
>> invoke slab shrinkers from shrink_zone()").
>>
>> So do not consider reclaimed slab pages for vmpressure calculation. The
>> reclaimed pages from slab can be excluded because the freeing of a page by
>> slab shrinking depends on each slab's object population, making the cost
>> model (i.e. scan:free) different from that of LRU. Also, not every
>> shrinker accounts the pages it reclaims. But ideally the pages reclaimed
>> from slab should be passed to vmpressure, otherwise higher vmpressure
>> levels can be triggered even when there is a reclaim progress. But
>> accounting only the reclaimed slab pages without the scanned, and adding
>> something which does not fit into the cost model just adds noise to the
>> vmpressure values.
> I believe there are still some of my questions which are not answered by
> the changelog update. Namely
> - vmstat numbers without mentioning vmpressure events for those 2
> kernels have basically no meaning.
Sending a new version. The vmpressure events difference is added.
> - the changelog doesn't mention that the test case basically benefits
> from as many lmk interventions as possible. Does this represent a real
> life workload? If not is there any real life workload which would
> benefit from the new behavior.
The use case does not actually benefit from as many lmk interventions as possible. Because it has to also take care
of maximizing the number of applications sustained. IMHO Android using a vmpressure based user space lowmemorykiller
is a real life workload. But the lowmemorykiller killer example was just to show the difference in vmpressure events between
2 kernel versions. Any workload which uses vmpressure would be something similar ? It would take an action by killing tasks,
or releasing some buffers etc as I understand. The patch was actually meant to fix the addition of noise to vmpressure by
adding reclaimed without accounting the cost and the lmk example was just to indicate the difference in vmpressure events.
> - I would be also very careful calling this a regression without having
> any real workload as an example
Okay. I have removed that from changelog.
> - Arguments about the cost model is are true but the resulting code is
> not a 100% win either and the changelog should be explicit about the
> consequences - aka more critical events can fire early while there is
> still slab making a reclaim progress.
>
This line was added to changelog indicating the consequence.
"Ideally the pages reclaimed from slab should be passed to vmpressure, otherwise higher vmpressure levels can
be triggered even when there is a reclaim progress."
Thanks,
Vinayak
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