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

Yu Zhao
2022-03-15T05:19:01Z

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".

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