On Mon, Oct 21, 2019 at 04:21:21PM +0200, Vitaly Wool wrote: > On Tue, Oct 15, 2019 at 10:00 PM Minchan Kim <minchan@xxxxxxxxxx> wrote: > > > > On Tue, Oct 15, 2019 at 09:39:35AM +0200, Vitaly Wool wrote: > > > Hi Minchan, > > > > > > On Mon, Oct 14, 2019 at 6:41 PM Minchan Kim <minchan@xxxxxxxxxx> wrote: > > > > > > > > On Thu, Oct 10, 2019 at 11:04:14PM +0300, Vitaly Wool wrote: > > > > > The coming patchset is a new take on the old issue: ZRAM can currently be used only with zsmalloc even though this may not be the optimal combination for some configurations. The previous (unsuccessful) attempt dates back to 2015 [1] and is notable for the heated discussions it has caused. > > > > > > > > > > The patchset in [1] had basically the only goal of enabling ZRAM/zbud combo which had a very narrow use case. Things have changed substantially since then, and now, with z3fold used widely as a zswap backend, I, as the z3fold maintainer, am getting requests to re-interate on making it possible to use ZRAM with any zpool-compatible backend, first of all z3fold. > > > > > > > > > > The preliminary results for this work have been delivered at Linux Plumbers this year [2]. The talk at LPC, though having attracted limited interest, ended in a consensus to continue the work and pursue the goal of decoupling ZRAM from zsmalloc. > > > > > > > > > > The current patchset has been stress tested on arm64 and x86_64 devices, including the Dell laptop I'm writing this message on now, not to mention several QEmu confugirations. > > > > > > > > > > [1] https://lkml.org/lkml/2015/9/14/356 > > > > > [2] https://linuxplumbersconf.org/event/4/contributions/551/ > > > > > > > > Please describe what's the usecase in real world, what's the benefit zsmalloc > > > > cannot fulfill by desgin and how it's significant. > > > > > > I'm not entirely sure how to interpret the phrase "the benefit > > > zsmalloc cannot fulfill by design" but let me explain. > > > First, there are multi multi core systems where z3fold can provide > > > better throughput. > > > > Please include number in the description with workload. > > Sure. So on an HMP 8-core ARM64 system with ZRAM, we run the following command: > fio --bs=4k --randrepeat=1 --randseed=100 --refill_buffers \ > --buffer_compress_percentage=50 --scramble_buffers=1 \ > --direct=1 --loops=15 --numjobs=4 --filename=/dev/block/zram0 \ > --name=seq-write --rw=write --stonewall --name=seq-read \ > --rw=read --stonewall --name=seq-readwrite --rw=rw --stonewall \ > --name=rand-readwrite --rw=randrw --stonewall > > The results are the following: > > zsmalloc: > Run status group 0 (all jobs): > WRITE: io=61440MB, aggrb=1680.4MB/s, minb=430167KB/s, > maxb=440590KB/s, mint=35699msec, maxt=36564msec > > Run status group 1 (all jobs): > READ: io=61440MB, aggrb=1620.4MB/s, minb=414817KB/s, > maxb=414850KB/s, mint=37914msec, maxt=37917msec > > Run status group 2 (all jobs): > READ: io=30615MB, aggrb=897979KB/s, minb=224494KB/s, > maxb=228161KB/s, mint=34351msec, maxt=34912msec > WRITE: io=30825MB, aggrb=904110KB/s, minb=226027KB/s, > maxb=229718KB/s, mint=34351msec, maxt=34912msec > > Run status group 3 (all jobs): > READ: io=30615MB, aggrb=772002KB/s, minb=193000KB/s, > maxb=193010KB/s, mint=40607msec, maxt=40609msec > WRITE: io=30825MB, aggrb=777273KB/s, minb=194318KB/s, > maxb=194327KB/s, mint=40607msec, maxt=40609msec > > z3fold: > Run status group 0 (all jobs): > WRITE: io=61440MB, aggrb=1224.8MB/s, minb=313525KB/s, > maxb=329941KB/s, mint=47671msec, maxt=50167msec > > Run status group 1 (all jobs): > READ: io=61440MB, aggrb=3119.3MB/s, minb=798529KB/s, > maxb=862883KB/s, mint=18228msec, maxt=19697msec > > Run status group 2 (all jobs): > READ: io=30615MB, aggrb=937283KB/s, minb=234320KB/s, > maxb=234334KB/s, mint=33446msec, maxt=33448msec > WRITE: io=30825MB, aggrb=943682KB/s, minb=235920KB/s, > maxb=235934KB/s, mint=33446msec, maxt=33448msec > > Run status group 3 (all jobs): > READ: io=30615MB, aggrb=829591KB/s, minb=207397KB/s, > maxb=210285KB/s, mint=37271msec, maxt=37790msec > WRITE: io=30825MB, aggrb=835255KB/s, minb=208813KB/s, > maxb=211721KB/s, mint=37271msec, maxt=37790msec > > So, z3fold is faster everywhere (including being *two* times faster on > read) except for sequential write which is the least important use > case in real world. No. write is also important because it affects reclaim speed. I ran fio on x86 with various compression sizes. left is zsmalloc. right is z3fold The operation order is seq-write rand-write seq-read rand-read mixed-seq mixed-rand trim mem_used - byte unit Last column mem_used is to indicate how many allocator used the memory to store compressed page 1) compression ratio 75 WRITE 2535 WRITE 1928 WRITE 2425 WRITE 1886 READ 6211 READ 5731 READ 6339 READ 6182 READ 1791 READ 1592 WRITE 1790 WRITE 1591 READ 1704 READ 1493 WRITE 1699 WRITE 1489 WRITE 984 WRITE 974 TRIM 984 TRIM 974 mem_used 29986816 mem_used 61239296 For every operation, zsmalloc is faster than z3fold. Even, it used the 1/2 memory compared to z3fold. 2) compression ratio 66 WRITE 2125 WRITE 1258 WRITE 2107 WRITE 1233 READ 5714 READ 5793 READ 5948 READ 6065 READ 1667 READ 1248 WRITE 1666 WRITE 1247 READ 1521 READ 1218 WRITE 1517 WRITE 1215 WRITE 943 WRITE 870 TRIM 943 TRIM 870 mem_used 38158336 mem_used 76779520 For only read operation, z3fold is a bit faster than zsmalloc about 2%. However, look at other operations which zsmalloc is much faster. Even, look at used memory. 3) compression ratio 50 WRITE 2051 WRITE 1109 WRITE 2029 WRITE 1087 READ 5366 READ 6364 READ 5575 READ 5785 READ 1497 READ 1121 WRITE 1496 WRITE 1121 READ 1432 READ 1065 WRITE 1428 WRITE 1062 WRITE 930 WRITE 838 TRIM 930 TRIM 838 mem_used 59932672 mem_used 104873984 sequential read on z3fold is faster about 15%. However, look at other operations and used memory. zsmalloc is better. Why zsmalloc is slow for 50% compression ratio is it needs page copy for every read operation since compressed objects cross over page boundary. However, I don't think it's real workload because compressed ratio will spread out into various sizes. Having said that, I could enhance zsmalloc to avoid the copy operation. I will work on it. 4) compression ratio 33 WRITE 1945 WRITE 1239 WRITE 1869 WRITE 1222 READ 5319 READ 6206 READ 5416 READ 6645 READ 1480 READ 1188 WRITE 1479 WRITE 1188 READ 1403 READ 1114 WRITE 1399 WRITE 1110 WRITE 930 WRITE 793 TRIM 930 TRIM 793 mem_used 78667776 mem_used 104873984 5) compression ratio 25 WRITE 1862 WRITE 1080 WRITE 1840 WRITE 1052 READ 5260 READ 6240 READ 5540 READ 6359 READ 1445 READ 1040 WRITE 1444 WRITE 1039 READ 1354 READ 1006 WRITE 1350 WRITE 1003 WRITE 909 WRITE 775 TRIM 909 TRIM 775 mem_used 83902464 mem_used 104873984 If compress ratio is bad, zram read operation with zsmalloc could be slower about 15% than z3fold because it needs additional memory copy as I mentioned. However, it's still faster if compression ratio ig greater than 50%, which is usual case(I believe that's why you makes z3fold). > > > > Then, there are low end systems with hardware > > > compression/decompression support which don't need zsmalloc > > > sophistication and would rather use zbud with ZRAM because the > > > compression ratio is relatively low. > > > > I couldn't imagine how it's bad with zsmalloc. Could you be more > > specific? > > > > > Finally, there are MMU-less systems targeting IOT and still running > > > Linux and having a compressed RAM disk is something that would help > > > these systems operate in a better way (for the benefit of the overall > > > Linux ecosystem, if you care about that, of course; well, some people > > > do). > > > > Could you write down what's the problem to use zsmalloc for MMU-less > > system? Maybe, it would be important point rather other performance > > argument since other functions's overheads in the callpath are already > > rather big. > > Well, I assume you had the reasons to make zsmalloc depend on MMU in Kconfig: > ... > config ZSMALLOC > tristate "Memory allocator for compressed pages" > depends on MMU > help > ... It's old piece left since zsmalloc used mapping API so I think we could remove the dependency now. However, I want to know it's the only problem to use zram in MMU-less system. IOW, if we could remove the zsmalloc MMU dependency, it's ready to use zram on MMU-less system now? > > But even disregarding that, let's compare ZRAM/zbud and ZRAM/zsmalloc > performance and memory these two consume on a relatively low end > 2-core ARM. > Command: > fio --bs=4k --randrepeat=1 --randseed=100 --refill_buffers > --scramble_buffers=1 \ > --direct=1 --loops=15 --numjobs=2 --filename=/dev/block/zram0 \ > --name=seq-write --rw=write --stonewall --name=seq-read --rw=read \ > --stonewall --name=seq-readwrite --rw=rw --stonewall > --name=rand-readwrite \ > --rw=randrw --stonewall > > zsmalloc: > Run status group 0 (all jobs): > WRITE: io=30720MB, aggrb=374763KB/s, minb=187381KB/s, > maxb=188389KB/s, mint=83490msec, maxt=83939msec > > Run status group 1 (all jobs): > READ: io=30720MB, aggrb=964000KB/s, minb=482000KB/s, > maxb=482015KB/s, mint=32631msec, maxt=32632msec > > Run status group 2 (all jobs): > READ: io=15308MB, aggrb=431263KB/s, minb=215631KB/s, > maxb=215898KB/s, mint=36302msec, maxt=36347msec > WRITE: io=15412MB, aggrb=434207KB/s, minb=217103KB/s, > maxb=217373KB/s, mint=36302msec, maxt=36347msec > > Run status group 3 (all jobs): > READ: io=15308MB, aggrb=327328KB/s, minb=163664KB/s, > maxb=163667KB/s, mint=47887msec, maxt=47888msec > WRITE: io=15412MB, aggrb=329563KB/s, minb=164781KB/s, > maxb=164785KB/s, mint=47887msec, maxt=47888msec > > zbud: > Run status group 0 (all jobs): > WRITE: io=30720MB, aggrb=735980KB/s, minb=367990KB/s, > maxb=373079KB/s, mint=42159msec, maxt=42742msec > > Run status group 1 (all jobs): > READ: io=30720MB, aggrb=927915KB/s, minb=463957KB/s, > maxb=463999KB/s, mint=33898msec, maxt=33901msec > > Run status group 2 (all jobs): > READ: io=15308MB, aggrb=403467KB/s, minb=201733KB/s, > maxb=202051KB/s, mint=38790msec, maxt=38851msec > WRITE: io=15412MB, aggrb=406222KB/s, minb=203111KB/s, > maxb=203430KB/s, mint=38790msec, maxt=38851msec > > Run status group 3 (all jobs): > READ: io=15308MB, aggrb=334967KB/s, minb=167483KB/s, > maxb=167487KB/s, mint=46795msec, maxt=46796msec > WRITE: io=15412MB, aggrb=337254KB/s, minb=168627KB/s, > maxb=168630KB/s, mint=46795msec, maxt=46796msec > > Pretty equal except for sequential write which is twice as good with zbud. Thanks for the testing. I also tried to test zbud with zram but failed because fio submit incompressible pages to zram even though it specifiy compress ratio 100% However, zbud doesn't support 4K page allocation so zram couldn't work on it at this moment. I tried various fio versions as well as old but everything failed. How did you test it successfully? Let me know your fio version. I want to investigate what's the performance bottleneck beside page copy so that I will optimize it. > > Now to the fun part. > zsmalloc: > 0 .text 00002908 0000000000000000 0000000000000000 00000040 2**2 > CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE > zbud: > 0 .text 0000072c 0000000000000000 0000000000000000 00000040 2**2 > CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE > > And this does not cover dynamic memory allocation overhead which is > higher for zsmalloc. So once again, given that the compression ratio > is low (e. g. a simple HW accelerator is used), what would most > unbiased people prefer to use in this case? Zsmalloc has more features than zbud. That's why you see the code size difference. It was intentional because at that time most of users were mobile phones, TV and other smart devices. They needed those features. We could make those feature turned off at build time, which will improve performance and reduce code size a lot. It would be no problem if the user wanted to use zbud which is alredy lacking of those features. > > > > > I really don't want to make fragmentaion of allocator so we should really see > > > > how zsmalloc cannot achieve things if you are claiming. > > > > > > I have to say that this point is completely bogus. We do not create > > > fragmentation by using a better defined and standardized API. In fact, > > > we aim to increase the number of use cases and test coverage for ZRAM. > > > With that said, I have hard time seeing how zsmalloc can operate on a > > > MMU-less system. > > > > > > > Please tell us how to test it so that we could investigate what's the root > > > > cause. > > > > > > I gather you haven't read neither the LPC documents nor my > > > conversation with Sergey re: these changes, because if you did you > > > wouldn't have had the type of questions you're asking. Please also see > > > above. > > > > Please include your claims in the description rather than attaching > > file. That's the usualy way how we work because it could make easier to > > discuss by inline. > > Did I attach something? I don't quite recall that. I posted links to > previous discussions and conference materials, each for a reason. > > > > > > > I feel a bit awkward explaining basic things to you but there may not > > > be other "root cause" than applicability issue. zsmalloc is a great > > > allocator but it's not universal and has its limitations. The > > > (potential) scope for ZRAM is wider than zsmalloc can provide. We are > > > *helping* _you_ to extend this scope "in real world" (c) and you come > > > up with bogus objections. Why? > > > > Please add more detail to convince so we need to think over why zsmalloc > > cannot be improved for the usecase. > > This approach is wrong. zsmalloc is good enough and covers a lot of > use cases but there are still some where it doesn't work that well by > design. E. g. on an XIP system we do care about the code size since > it's stored uncompressed but still want to use ZRAM. Why would we want > to waste almost 10K just on zsmalloc code if the counterpart (zbud in > that case) works better? As I mentiond, we could improve zsmalloc to reduce code size as well as performance. I will work on it.
