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

Barry Song
2022-03-15T09:27:41Z

On Tue, Mar 15, 2022 at 6:18 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Mon, Mar 14, 2022 at 5:38 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > On Tue, Mar 15, 2022 at 5:45 AM Yu Zhao <yuzhao@google.com> wrote:
> > >
> > > On Mon, Mar 14, 2022 at 5:12 AM Barry Song <21cnbao@gmail.com> wrote:
> > > >
> > > > > > > >
> > > > > > > > > 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.
> > >
> > > You've missed something very important: *thoughput* :)
> > >
> >
> > noop :-)
> > in the test case, there are 4 threads. they are searching a key in 10 chunks
> > of memory. for each chunk, the size is 200MB.
> > a "random" chunk index is returned for those threads to search. but chunk2
> > is the hottest, and chunk3, 7, 4 are relatively hotter than others.
> > static inline unsigned int rand_chunk(void)
> > {
> > /* simulate hot and cold chunk */
> > unsigned int rand[16] = {2, 2, 3, 4, 5, 2, 6, 7, 9, 2, 8, 3, 7, 2, 2, 4};
>
> This is sequential access, not what you claim above, because you have
> a repeating sequence.
>
> In this case MGLRU is expected to be slower because it doesn't try to
> optimize it, as discussed before [1]. The reason is, with a manageable
> complexity, we can only optimize so many things. And MGLRU chose to
> optimize (arguably) popular workloads, since, AFAIK, no real-world
> applications streams anon memory.
>
> To verify this is indeed sequential access, you could make rand[]
> larger, e.g., 160, with the same portions of 2s, 3s, 4s, etc, but
> their positions are random. The following change shows MGLRU is ~20%
> faster on my Snapdragon 7c + 2.5G DRAM + 2GB zram.
>
> static inline unsigned int rand_chunk(void)
> {
> /* simulate hot and cold chunk */
> - unsigned int rand[16] = {2, 2, 3, 4, 5, 2, 6, 7, 9, 2, 8, 3,
> 7, 2, 2, 4};
> + unsigned int rand[160] = {
> + 2, 4, 7, 3, 4, 2, 7, 2, 7, 8, 6, 9, 7, 6, 5, 4,
> + 6, 2, 6, 4, 2, 9, 2, 5, 5, 4, 7, 2, 7, 7, 5, 2,
> + 4, 4, 3, 3, 2, 4, 2, 2, 5, 2, 4, 2, 8, 2, 2, 3,
> + 2, 2, 2, 2, 2, 8, 4, 2, 2, 4, 2, 2, 2, 2, 3, 2,
> + 8, 5, 2, 2, 3, 2, 8, 2, 6, 2, 4, 8, 5, 2, 9, 2,
> + 8, 7, 9, 2, 4, 4, 3, 3, 2, 8, 2, 2, 3, 3, 2, 7,
> + 7, 5, 2, 2, 8, 2, 2, 2, 5, 2, 4, 3, 2, 3, 6, 3,
> + 3, 3, 9, 4, 2, 3, 9, 7, 7, 6, 2, 2, 4, 2, 6, 2,
> + 9, 7, 7, 7, 9, 3, 4, 2, 3, 2, 7, 3, 2, 2, 2, 6,
> + 8, 3, 7, 6, 2, 2, 2, 4, 7, 2, 5, 7, 4, 7, 9, 9,
> + };
> static int nr = 0;
> - return rand[nr++%16];
> + return rand[nr++%160];
> }
>
> Yet better, you could use some standard benchmark suites, written by
> reputable organizations, e.g., memtier, YCSB, to generate more
> realistic distributions, as I've suggested before [2].
>
> > static int nr = 0;
> > return rand[nr++%16];
> > }
> >
> > each thread does search_mem():
> > static unsigned int search_mem(void)
> > {
> > record_t key, *found;
> > record_t *src, *copy;
> > unsigned int chunk;
> > size_t copy_size = chunk_size;
> > unsigned int i;
> > unsigned int state = 0;
> >
> > /* run 160 loops or till timeout */
> > for (i = 0; threads_go == 1 && i < 160; i++) {
>
> I see you've modified the original benchmark. But with "-S 200000",
> should this test finish within an hour instead of the following?
> Elapsed (wall clock) time (h:mm:ss or m:ss): 1:11.59
>
> > chunk = rand_chunk();
> > src = mem[chunk];
> > ...
> > copy = alloc_mem(copy_size);
> > ...
> > memcpy(copy, src, copy_size);
> >
> > key = rand_num(copy_size / record_size, &state);
> >
> > bsearch(&key, copy, copy_size / record_size,
> > record_size, compare);
> >
> > /* Below check is mainly for memory corruption or other bug */
> > if (found == NULL) {
> > fprintf(stderr, "Couldn't find key %zd\n", key);
> > exit(1);
> > }
> > } /* end if ! touch_pages */
> >
> > free_mem(copy, copy_size);
> > }
> >
> > return (i);
> > }
> >
> > each thread picks up a chunk, then allocates a new memory and copies the chunk to the
> > new allocated memory, and searches a key in the allocated memory.
> >
> > as i have set time to rather big by -S, so each thread actually exits while it
> > completes 160 loops.
> > $ \time -v ./ebizzy -t 4 -s $((200*1024*1024)) -S 6000000
>
> Ok, you actually used "-S 6000000".

I have two exits, either 160 loops have been done or -S gets timeout.
Since -S is very big, the process exits from the completion of 160
loops.

I am seeing mglru is getting very similar speed with vanilla lru by
using your rand_chunk() with 160 entries. the command is like:
\time -v ./a.out -t 4 -s $((200*1024*1024)) -S 600000 -m

The time to complete jobs begins to be more random, but on average,
mglru seems to be 5% faster. actually, i am seeing mglru can be faster
than vanilla even with more page faults. for example,

MGLRU:
Command being timed: "./mt.out -t 4 -s 209715200 -S 600000 -m"
User time (seconds): 32.68
System time (seconds): 227.19
Percent of CPU this job got: 370%
Elapsed (wall clock) time (h:mm:ss or m:ss): 1:10.23
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): 2175292
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 10977244
Minor (reclaiming a frame) page faults: 33447638
Voluntary context switches: 44466
Involuntary context switches: 108413
Swaps: 0
File system inputs: 7704
File system outputs: 8
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

VANILLA:
Command being timed: "./mt.out -t 4 -s 209715200 -S 600000 -m"
User time (seconds): 32.20
System time (seconds): 248.18
Percent of CPU this job got: 371%
Elapsed (wall clock) time (h:mm:ss or m:ss): 1:15.55
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): 2174384
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 10002206
Minor (reclaiming a frame) page faults: 33392151
Voluntary context switches: 76966
Involuntary context switches: 184841
Swaps: 0
File system inputs: 2032
File system outputs: 8
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

I guess the main cause of the regression for the previous sequence
with 16 entries is that the ebizzy has a new allocated copy in
search_mem(), which is mapped and used only once in each loop.
and the temp copy can push out those hot chunks.

Anyway, I understand it is a trade-off between warmly embracing new
pages and holding old pages tightly. Real user cases from phone, server,
desktop will be judging this better.

>
> [1] https://lore.kernel.org/linux-mm/YhNJ4LVWpmZgLh4I@google.com/
> [2] https://lore.kernel.org/linux-mm/YgggI+vvtNvh3jBY@google.com/

Thanks
Barry