Re: [PATCH v7 04/12] mm: multigenerational LRU: groundwork

[Barry Song <21cnbao@gmail.com>]

> > > >
> > > > > We used to put a faulted file page in inactive, if we access it a
> > > > > second time, it can be promoted
> > > > > to active. then in recent years, we have also applied this to anon
> > > > > pages while kernel adds
> > > > > workingset protection for anon pages. so basically both anon and file
> > > > > pages go into the inactive
> > > > > list for the 1st time, if we access it for the second time, they go to
> > > > > the active list. if we don't access
> > > > > it any more, they are likely to be reclaimed as they are inactive.
> > > > > we do have some special fastpath for code section, executable file
> > > > > pages are kept on active list
> > > > > as long as they are accessed.
> > > >
> > > > Yes.
> > > >
> > > > > so all of the above concerns are actually not that correct?
> > > >
> > > > They are valid concerns but I don't know any popular workloads that
> > > > care about them.
> > >
> > > Hi Yu,
> > > here we can get a workload in Kim's patchset while he added workingset
> > > protection
> > > for anon pages:
> > > https://patchwork.kernel.org/project/linux-mm/cover/1581401993-20041-1-git-send-email-iamjoonsoo.kim@lge.com/
> >
> > Thanks. I wouldn't call that a workload because it's not a real
> > application. By popular workloads, I mean applications that the
> > majority of people actually run on phones, in cloud, etc.
> >
> > > anon pages used to go to active rather than inactive, but kim's patchset
> > > moved to use inactive first. then only after the anon page is accessed
> > > second time, it can move to active.
> >
> > Yes. To clarify, the A-bit doesn't really mean the first or second
> > access. It can be many accesses each time it's set.
> >
> > > "In current implementation, newly created or swap-in anonymous page is
> > >
> > > started on the active list. Growing the active list results in rebalancing
> > > active/inactive list so old pages on the active list are demoted to the
> > > inactive list. Hence, hot page on the active list isn't protected at all.
> > >
> > > Following is an example of this situation.
> > >
> > > Assume that 50 hot pages on active list and system can contain total
> > > 100 pages. Numbers denote the number of pages on active/inactive
> > > list (active | inactive). (h) stands for hot pages and (uo) stands for
> > > used-once pages.
> > >
> > > 1. 50 hot pages on active list
> > > 50(h) | 0
> > >
> > > 2. workload: 50 newly created (used-once) pages
> > > 50(uo) | 50(h)
> > >
> > > 3. workload: another 50 newly created (used-once) pages
> > > 50(uo) | 50(uo), swap-out 50(h)
> > >
> > > As we can see, hot pages are swapped-out and it would cause swap-in later."
> > >
> > > Is MGLRU able to avoid the swap-out of the 50 hot pages?
> >
> > I think the real question is why the 50 hot pages can be moved to the
> > inactive list. If they are really hot, the A-bit should protect them.
>
> This is a good question.
>
> I guess it  is probably because the current lru is trying to maintain a balance
> between the sizes of active and inactive lists. Thus, it can shrink active list
> even though pages might be still "hot" but not the recently accessed ones.
>
> 1. 50 hot pages on active list
> 50(h) | 0
>
> 2. workload: 50 newly created (used-once) pages
> 50(uo) | 50(h)
>
> 3. workload: another 50 newly created (used-once) pages
> 50(uo) | 50(uo), swap-out 50(h)
>
> the old kernel without anon workingset protection put workload 2 on active, so
> pushed 50 hot pages from active to inactive. workload 3 would further contribute
> to evict the 50 hot pages.
>
> it seems mglru doesn't demote pages from the youngest generation to older
> generation only in order to balance the list size? so mglru is probably safe
> in these cases.
>
> I will run some tests mentioned in Kim's patchset and report the result to you
> afterwards.
>

Hi Yu,
I did find putting faulted pages to the youngest generation lead to some
regression in the case ebizzy Kim's patchset mentioned while he tried
to support workingset protection for anon pages.
i did a little bit modification for rand_chunk() which is probably similar
with the modifcation() Kim mentioned in his patchset. The modification
can be found here:
https://github.com/21cnbao/ltp/commit/7134413d747bfa9ef

The test env is a x86 machine in which I have set memory size to 2.5GB and
set zRAM to 2GB and disabled external disk swap.

with the vanilla kernel:
\time -v ./a.out -vv -t 4 -s 209715200 -S 200000

so we have 10 chunks and 4 threads, each trunk is 209715200(200MB)

typical result:
        Command being timed: "./a.out -vv -t 4 -s 209715200 -S 200000"
        User time (seconds): 36.19
        System time (seconds): 229.72
        Percent of CPU this job got: 371%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 1:11.59
        Average shared text size (kbytes): 0
        Average unshared data size (kbytes): 0
        Average stack size (kbytes): 0
        Average total size (kbytes): 0
        Maximum resident set size (kbytes): 2166196
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 9990128
        Minor (reclaiming a frame) page faults: 33315945
        Voluntary context switches: 59144
        Involuntary context switches: 167754
        Swaps: 0
        File system inputs: 2760
        File system outputs: 8
        Socket messages sent: 0
        Socket messages received: 0
        Signals delivered: 0
        Page size (bytes): 4096
        Exit status: 0

with gen_lru and lru_gen/enabled=0x3:
typical result:
Command being timed: "./a.out -vv -t 4 -s 209715200 -S 200000"
User time (seconds): 36.34
System time (seconds): 276.07
Percent of CPU this job got: 378%
Elapsed (wall clock) time (h:mm:ss or m:ss): 1:22.46
           **** 15% time +
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 2168120
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 13362810
             ***** 30% page fault +
Minor (reclaiming a frame) page faults: 33394617
Voluntary context switches: 55216
Involuntary context switches: 137220
Swaps: 0
File system inputs: 4088
File system outputs: 8
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

with gen_lru and lru_gen/enabled=0x7:
typical result:
Command being timed: "./a.out -vv -t 4 -s 209715200 -S 200000"
User time (seconds): 36.13
System time (seconds): 251.71
Percent of CPU this job got: 378%
Elapsed (wall clock) time (h:mm:ss or m:ss): 1:16.00
         *****better than enabled=0x3, worse than vanilla
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 2120988
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 12706512
Minor (reclaiming a frame) page faults: 33422243
Voluntary context switches: 49485
Involuntary context switches: 126765
Swaps: 0
File system inputs: 2976
File system outputs: 8
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

I can also reproduce the problem on arm64.

I am not saying this is going to block mglru from being mainlined. But  I am
still curious if this is an issue worth being addressed somehow in mglru.

> >
> > > since MGLRU
> > > is putting faulted pages to the youngest generation directly, do we have the
> > > risk mentioned in Kim's patchset?
> >
> > There are always risks :) I could imagine a thousand ways to make VM
> > suffer, but all of them could be irrelevant to how it actually does in
> > production. So a concrete use case of yours would be much appreciated
> > for this discussion.
>

Thanks
Barry