Re: [PoC] Non-volatile WAL buffer

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(dot)mshk(at)gmail(dot)com>:

> On Fri, Jan 22, 2021 at 11:32 AM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
> >
> >
> >
> > On 1/21/21 3:17 AM, Masahiko Sawada wrote:
> > > On Thu, Jan 7, 2021 at 2:16 AM Tomas Vondra
> > > <tomas(dot)vondra(at)enterprisedb(dot)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(dot)menjo(at)gmail(dot)com>