Re: walwriter interacts quite badly with synchronous_commit=off

On 25/10/2023 02:09, Andres Freund wrote:
> Because of the inherent delay between the checks of XLogCtl->WalWriterSleeping
> and Latch->is_set, we also sometimes end up with multiple processes signalling
> walwriter, which can be bad, because it increases the likelihood that some of
> the signals may be received when we are already holding WALWriteLock, delaying
> its release...

That can only happen when walwriter has just come out of "hibernation",
ie. when the system has been idle for a while. So probably not a big
deal in practice.

> I think the most important optimization we need is to have
> XLogSetAsyncXactLSN() only wake up if there is a certain amount of unflushed
> WAL. Unless walsender is hibernating, walsender will wake up on its own after
> wal_writer_delay. I think we can just reuse WalWriterFlushAfter for this.
>
> E.g. a condition like
> if (WriteRqstPtr <= LogwrtResult.Write + WalWriterFlushAfter * XLOG_BLCKSZ)
> return;
> drastically cuts down on the amount of wakeups, without - I think - loosing
> guarantees around synchronous_commit=off.

In the patch, you actually did:

> + if (WriteRqstPtr <= LogwrtResult.Flush + WalWriterFlushAfter * XLOG_BLCKSZ)
> + return;

It means that you never wake up the walwriter to merely *write* the WAL.
You only wake it up if it's going to also fsync() it. I think that's
correct and appropriate, but it took me a while to reach that conclusion:

It might be beneficial to wake up the walwriter just to perform a
write(), to offload that work from the backend. And walwriter will
actually also perform an fsync() after finishing the current segment, so
it would make sense to also wake it up when 'asyncXactLSN' crosses a
segment boundary. However, if those extra wakeups make sense, they would
also make sense when there are no asynchronous commits involved.
Therefore those extra wakeups should be done elsewhere, perhaps
somewhere around AdvanceXLInsertBuffer(). The condition you have in the
patch is appropriate for XLogSetAsyncXactLSN().

Another reason to write the WAL aggressively, even if you don't flush
it, would be to reduce the number of lost transactions on a segfault.
But we don't give any guarantees on that, and even with the current
aggressive logic, we only write when a page is full so you're anyway
going to lose the last partial page.

It also took me a while to convince myself that this calculation matches
the calculation that XLogBackgroundFlush() uses to determine whether it
needs to flush or not. XLogBackgroundFlush() first divides the request
and result with XLOG_BLCKSZ and then compares the number of blocks,
whereas here you perform the calculation in bytes. I think the result is
the same, but to make it more clear, let's do it the same way in both
places.

See attached. It's the same logic as in your patch, just formulatd more
clearly IMHO.

> Because of the frequent wakeups, we do something else that's not smart: We
> write out 8k blocks individually, many times a second. Often thousands of
> 8k pwrites a second.

Even with this patch, when I bumped up wal_writer_delay to 2 so that the
wal writer gets woken up by the async commits rather than the timeout,
the write pattern is a bit silly:

$ strace -p 1099926 # walwriter
strace: Process 1099926 attached
epoll_wait(10, [{events=EPOLLIN, data={u32=3704011232,
u64=94261056289248}}], 1, 1991) = 1
read(3,
"\27\0\0\0\0\0\0\0\0\0\0\0<\312\20\0\350\3\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
1024) = 128
pwrite64(5,
"\24\321\5\0\1\0\0\0\0\300\0\373\5\0\0\0+\0\0\0\0\0\0\0\0\n\0\0n\276\242\305"...,
1007616, 49152) = 1007616
fdatasync(5) = 0
pwrite64(5, "\24\321\5\0\1\0\0\0\0
\20\373\5\0\0\0003\0\0\0\0\0\0\0\320\37\20\373\5\0\0\0"..., 16384,
1056768) = 16384
epoll_wait(10, [{events=EPOLLIN, data={u32=3704011232,
u64=94261056289248}}], 1, 2000) = 1
read(3,
"\27\0\0\0\0\0\0\0\0\0\0\0<\312\20\0\350\3\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
1024) = 128
pwrite64(5,
"\24\321\5\0\1\0\0\0\0`\20\373\5\0\0\0+\0\0\0\0\0\0\0\0\n\0\0\5~\23\261"...,
1040384, 1073152) = 1040384
fdatasync(5) = 0
pwrite64(5, "\24\321\4\0\1\0\0\0\0@
\373\5\0\0\0\0\0\0\0\0\0\0\0;\0\0\0\264'\246\3"..., 16384, 2113536) = 16384
epoll_wait(10, [{events=EPOLLIN, data={u32=3704011232,
u64=94261056289248}}], 1, 2000) = 1
read(3,
"\27\0\0\0\0\0\0\0\0\0\0\0<\312\20\0\350\3\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
1024) = 128
pwrite64(5, "\24\321\5\0\1\0\0\0\0\200
\373\5\0\0\0003\0\0\0\0\0\0\0\320\177 \373\5\0\0\0"..., 1040384,
2129920) = 1040384
fdatasync(5) = 0

In each cycle, the wal writer writes a full 1 MB chunk
(wal_writer_flush_after = '1MB'), flushes it, and then perform a smaller
write before going to sleep.

Those smaller writes seem a bit silly. But I think it's fine.

> More throughput for less CPU, seems neat :)

Indeed, impressive speedup from such a small patch!

> I'm not addressing that here, but I think we also have the opposite behaviour
> - we're not waking up walwriter often enough. E.g. if you have lots of bulk
> dataloads, walwriter will just wake up once per wal_writer_delay, leading to
> most of the work being done by backends. We should probably wake walsender at
> the end of XLogInsertRecord() if there is sufficient outstanding WAL.

Right, that's basically the same issue that I reasoned through above. I
did some quick testing with a few different settings of wal_buffers,
wal_writer_flush_after and wal_writer_delay, to try to see that effect.
But I was not able to find a case where it makes a difference.

--
Heikki Linnakangas
Neon (https://neon.tech)