Dear everyone, I'm sorry for the late reply. I rebase my two patchsets onto the latest master 411ae64.The one patchset prefixed with v4 is for non-volatile WAL buffer; the other prefixed with v3 is for msync.
I will reply to your thankful feedbacks one by one within days. Please wait for a moment. Best regards, Takashi 01/25/2021(Mon) 11:56 Masahiko Sawada <sawada.m...@gmail.com>: > On Fri, Jan 22, 2021 at 11:32 AM Tomas Vondra > <tomas.von...@enterprisedb.com> wrote: > > > > > > > > On 1/21/21 3:17 AM, Masahiko Sawada wrote: > > > On Thu, Jan 7, 2021 at 2:16 AM Tomas Vondra > > > <tomas.von...@enterprisedb.com> wrote: > > >> > > >> Hi, > > >> > > >> I think I've managed to get the 0002 patch [1] rebased to master and > > >> working (with help from Masahiko Sawada). It's not clear to me how it > > >> could have worked as submitted - my theory is that an incomplete patch > > >> was submitted by mistake, or something like that. > > >> > > >> Unfortunately, the benchmark results were kinda disappointing. For a > > >> pgbench on scale 500 (fits into shared buffers), an average of three > > >> 5-minute runs looks like this: > > >> > > >> branch 1 16 32 64 96 > > >> ---------------------------------------------------------------- > > >> master 7291 87704 165310 150437 224186 > > >> ntt 7912 106095 213206 212410 237819 > > >> simple-no-buffers 7654 96544 115416 95828 103065 > > >> > > >> NTT refers to the patch from September 10, pre-allocating a large WAL > > >> file on PMEM, and simple-no-buffers is the simpler patch simply > removing > > >> the WAL buffers and writing directly to a mmap-ed WAL segment on PMEM. > > >> > > >> Note: The patch is just replacing the old implementation with mmap. > > >> That's good enough for experiments like this, but we probably want to > > >> keep the old one for setups without PMEM. But it's good enough for > > >> testing, benchmarking etc. > > >> > > >> Unfortunately, the results for this simple approach are pretty bad. > Not > > >> only compared to the "ntt" patch, but even to master. I'm not entirely > > >> sure what's the root cause, but I have a couple hypotheses: > > >> > > >> 1) bug in the patch - That's clearly a possibility, although I've > tried > > >> tried to eliminate this possibility. > > >> > > >> 2) PMEM is slower than DRAM - From what I know, PMEM is much faster > than > > >> NVMe storage, but still much slower than DRAM (both in terms of > latency > > >> and bandwidth, see [2] for some data). It's not terrible, but the > > >> latency is maybe 2-3x higher - not a huge difference, but may matter > for > > >> WAL buffers? > > >> > > >> 3) PMEM does not handle parallel writes well - If you look at [2], > > >> Figure 4(b), you'll see that the throughput actually *drops" as the > > >> number of threads increase. That's pretty strange / annoying, because > > >> that's how we write into WAL buffers - each thread writes it's own > data, > > >> so parallelism is not something we can get rid of. > > >> > > >> I've added some simple profiling, to measure number of calls / time > for > > >> each operation (use -DXLOG_DEBUG_STATS to enable). It accumulates data > > >> for each backend, and logs the counts every 1M ops. > > >> > > >> Typical stats from a concurrent run looks like this: > > >> > > >> xlog stats cnt 43000000 > > >> map cnt 100 time 5448333 unmap cnt 100 time 3730963 > > >> memcpy cnt 985964 time 1550442272 len 15150499 > > >> memset cnt 0 time 0 len 0 > > >> persist cnt 13836 time 10369617 len 16292182 > > >> > > >> The times are in nanoseconds, so this says the backend did 100 mmap > and > > >> unmap calls, taking ~10ms in total. There were ~14k pmem_persist > calls, > > >> taking 10ms in total. And the most time (~1.5s) was used by > pmem_memcpy > > >> copying about 15MB of data. That's quite a lot :-( > > > > > > It might also be interesting if we can see how much time spent on each > > > logging function, such as XLogInsert(), XLogWrite(), and XLogFlush(). > > > > > > > Yeah, we could extend it to that, that's fairly mechanical thing. Bbut > > maybe that could be visible in a regular perf profile. Also, I suppose > > most of the time will be used by the pmem calls, shown in the stats. > > > > >> > > >> My conclusion from this is that eliminating WAL buffers and writing > WAL > > >> directly to PMEM (by memcpy to mmap-ed WAL segments) is probably not > the > > >> right approach. > > >> > > >> I suppose we should keep WAL buffers, and then just write the data to > > >> mmap-ed WAL segments on PMEM. Which I think is what the NTT patch > does, > > >> except that it allocates one huge file on PMEM and writes to that > > >> (instead of the traditional WAL segments). > > >> > > >> So I decided to try how it'd work with writing to regular WAL > segments, > > >> mmap-ed ad hoc. The pmem-with-wal-buffers-master.patch patch does > that, > > >> and the results look a bit nicer: > > >> > > >> branch 1 16 32 64 96 > > >> ---------------------------------------------------------------- > > >> master 7291 87704 165310 150437 224186 > > >> ntt 7912 106095 213206 212410 237819 > > >> simple-no-buffers 7654 96544 115416 95828 103065 > > >> with-wal-buffers 7477 95454 181702 140167 214715 > > >> > > >> So, much better than the version without WAL buffers, somewhat better > > >> than master (except for 64/96 clients), but still not as good as NTT. > > >> > > >> At this point I was wondering how could the NTT patch be faster when > > >> it's doing roughly the same thing. I'm sire there are some > differences, > > >> but it seemed strange. The main difference seems to be that it only > maps > > >> one large file, and only once. OTOH the alternative "simple" patch > maps > > >> segments one by one, in each backend. Per the debug stats the > map/unmap > > >> calls are fairly cheap, but maybe it interferes with the memcpy > somehow. > > >> > > > > > > While looking at the two methods: NTT and simple-no-buffer, I realized > > > that in XLogFlush(), NTT patch flushes (by pmem_flush() and > > > pmem_drain()) WAL without acquiring WALWriteLock whereas > > > simple-no-buffer patch acquires WALWriteLock to do that > > > (pmem_persist()). I wonder if this also affected the performance > > > differences between those two methods since WALWriteLock serializes > > > the operations. With PMEM, multiple backends can concurrently flush > > > the records if the memory region is not overlapped? If so, flushing > > > WAL without WALWriteLock would be a big benefit. > > > > > > > That's a very good question - it's quite possible the WALWriteLock is > > not really needed, because the processes are actually "writing" the WAL > > directly to PMEM. So it's a bit confusing, because it's only really > > concerned about making sure it's flushed. > > > > And yes, multiple processes certainly can write to PMEM at the same > > time, in fact it's a requirement to get good throughput I believe. My > > understanding is we need ~8 processes, at least that's what I heard from > > people with more PMEM experience. > > Thanks, that's good to know. > > > > > TBH I'm not convinced the code in the "simple-no-buffer" code (coming > > from the 0002 patch) is actually correct. Essentially, consider the > > backend needs to do a flush, but does not have a segment mapped. So it > > maps it and calls pmem_drain() on it. > > > > But does that actually flush anything? Does it properly flush changes > > done by other processes that may not have called pmem_drain() yet? I > > find this somewhat suspicious and I'd bet all processes that did write > > something have to call pmem_drain(). > > Yeah, in terms of experiments at least it's good to find out that the > approach mmapping each WAL segment is not good at performance. > > > > > > > >> So I did an experiment by increasing the size of the WAL segments. I > > >> chose to try with 521MB and 1024MB, and the results with 1GB look > like this: > > >> > > >> branch 1 16 32 64 96 > > >> ---------------------------------------------------------------- > > >> master 6635 88524 171106 163387 245307 > > >> ntt 7909 106826 217364 223338 242042 > > >> simple-no-buffers 7871 101575 199403 188074 224716 > > >> with-wal-buffers 7643 101056 206911 223860 261712 > > >> > > >> So yeah, there's a clear difference. It changes the values for > "master" > > >> a bit, but both the "simple" patches (with and without) WAL buffers > are > > >> much faster. The with-wal-buffers is almost equal to the NTT patch, > > >> which was using 96GB file. I presume larger WAL segments would get > even > > >> closer, if we supported them. > > >> > > >> I'll continue investigating this, but my conclusion so far seem to be > > >> that we can't really replace WAL buffers with PMEM - that seems to > > >> perform much worse. > > >> > > >> The question is what to do about the segment size. Can we reduce the > > >> overhead of mmap-ing individual segments, so that this works even for > > >> smaller WAL segments, to make this useful for common instances (not > > >> everyone wants to run with 1GB WAL). Or whether we need to adopt the > > >> design with a large file, mapped just once. > > >> > > >> Another question is whether it's even worth the extra complexity. On > > >> 16MB segments the difference between master and NTT patch seems to be > > >> non-trivial, but increasing the WAL segment size kinda reduces that. > So > > >> maybe just using File I/O on PMEM DAX filesystem seems good enough. > > >> Alternatively, maybe we could switch to libpmemblk, which should > > >> eliminate the filesystem overhead at least. > > > > > > I think the performance improvement by NTT patch with the 16MB WAL > > > segment, the most common WAL segment size, is very good (150437 vs. > > > 212410 with 64 clients). But maybe evaluating writing WAL segment > > > files on PMEM DAX filesystem is also worth, as you mentioned, if we > > > don't do that yet. > > > > > > > Well, not sure. I think the question is still open whether it's actually > > safe to run on DAX, which does not have atomic writes of 512B sectors, > > and I think we rely on that e.g. for pg_config. But maybe for WAL that's > > not an issue. > > I think we can use the Block Translation Table (BTT) driver that > provides atomic sector updates. > > > > > > Also, I'm interested in why the through-put of NTT patch saturated at > > > 32 clients, which is earlier than the master's one (96 clients). How > > > many CPU cores are there on the machine you used? > > > > > > > From what I know, this is somewhat expected for PMEM devices, for a > > bunch of reasons: > > > > 1) The memory bandwidth is much lower than for DRAM (maybe ~10-20%), so > > it takes fewer processes to saturate it. > > > > 2) Internally, the PMEM has a 256B buffer for writes, used for combining > > etc. With too many processes sending writes, it becomes to look more > > random, which is harmful for throughput. > > > > When combined, this means the performance starts dropping at certain > > number of threads, and the optimal number of threads is rather low > > (something like 5-10). This is very different behavior compared to DRAM. > > Makes sense. > > > > > There's a nice overview and measurements in this paper: > > > > Building blocks for persistent memory / How to get the most out of your > > new memory? > > Alexander van Renen, Lukas Vogel, Viktor Leis, Thomas Neumann & Alfons > > Kemper > > > > https://link.springer.com/article/10.1007/s00778-020-00622-9 > > Thank you. I'll read it. > > > > > > > >> I'm also wondering if WAL is the right usage for PMEM. Per [2] > there's a > > >> huge read-write assymmetry (the writes being way slower), and their > > >> recommendation (in "Observation 3" is) > > >> > > >> The read-write asymmetry of PMem im-plies the necessity of > avoiding > > >> writes as much as possible for PMem. > > >> > > >> So maybe we should not be trying to use PMEM for WAL, which is pretty > > >> write-heavy (and in most cases even write-only). > > > > > > I think using PMEM for WAL is cost-effective but it leverages the only > > > low-latency (sequential) write, but not other abilities such as > > > fine-grained access and low-latency random write. If we want to > > > exploit its all ability we might need some drastic changes to logging > > > protocol while considering storing data on PMEM. > > > > > > > True. I think investigating whether it's sensible to use PMEM for this > > purpose. It may turn out that replacing the DRAM WAL buffers with writes > > directly to PMEM is not economical, and aggregating data in a DRAM > > buffer is better :-( > > Yes. I think it might be interesting to do an analysis of the > bottlenecks of NTT patch by perf etc. If bottlenecks are moved to > other places by removing WALWriteLock during flush, it's probably a > good sign for further performance improvements. IIRC WALWriteLock is > one of the main bottlenecks on OLTP workload, although my memory might > already be out of date. > > Regards, > > -- > Masahiko Sawada > EDB: https://www.enterprisedb.com/ > -- Takashi Menjo <takashi.me...@gmail.com>
