On Tue, Mar 15, 2022 at 6:18 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > On Mon, Mar 14, 2022 at 5:38 PM Barry Song <21cnbao@xxxxxxxxx> wrote: > > > > On Tue, Mar 15, 2022 at 5:45 AM Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > > > > > On Mon, Mar 14, 2022 at 5:12 AM Barry Song <21cnbao@xxxxxxxxx> 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@xxxxxxx/ > > > > > > > > > > > > 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@xxxxxxxxxx/ > [2] https://lore.kernel.org/linux-mm/YgggI+vvtNvh3jBY@xxxxxxxxxx/ Thanks Barry
