On Sun, Aug 7, 2022 at 9:22 PM Bharath Rupireddy <bharath.rupireddyforpostg...@gmail.com> wrote: > > On Sun, Aug 7, 2022 at 3:19 PM Thomas Munro <thomas.mu...@gmail.com> wrote: > > > > > A second thing is that pg_pwritev_with_retry_and_write_zeros() is > > > designed to work on WAL segments initialization and it uses > > > XLOG_BLCKSZ and PGAlignedXLogBlock for the job, but there is nothing > > > in its name that tells us so. This makes me question whether > > > file_utils.c is a good location for this second thing. Could a new > > > file be a better location? We have a xlogutils.c in the backend, and > > > a name similar to that in src/common/ would be one possibility. > > > > Yeah, I think it should probably be disconnected from XLOG_BLCKSZ, or > > maybe it's OK to use BLCKSZ with a comment to say that it's a bit > > arbitrary, or maybe it's better to define a new zero buffer of some > > arbitrary size just in this code if that is too strange. We could > > experiment with different size buffers to see how it performs, bearing > > in mind that every time we double it you halve the number of system > > calls, but also bearing in mind that at some point it's too much for > > the stack. I can tell you that the way that code works today was not > > really written with performance in mind (unlike, say, the code > > reverted from 9.4 that tried to do this with posix_fallocate()), it > > was just finding an excuse to call pwritev(), to exercise new fallback > > code being committed for use by later AIO stuff (more patches coming > > soon). The retry support was added because it seemed plausible that > > some system out there would start to do short writes as we cranked up > > the sizes for some implementation reason other than ENOSPC, so we > > should make a reusable retry routine. > > Yes, doubling the zerobuffer size to say 2 * XLOG_BLCKSZ or 2 * BLCKSZ > reduces the system calls to half (right now, pg_pwritev_with_retry() > gets called 64 times per 16MB WAL file, it writes in the batches of 32 > blocks per call). > > Is there a ready-to-use tool or script or specific settings for > pgbench (pgbench command line options or GUC settings) that I can play > with to measure the performance?
I played with a simple insert use-case [1] that generates ~380 WAL files, with different block sizes. To my surprise, I have not seen any improvement with larger block sizes. I may be doing something wrong here, suggestions on to test and see the benefits are welcome. > > I think this should also handle the remainder after processing whole > > blocks, just for completeness. If I call the code as presented with size > > 8193, I think this code will only write 8192 bytes. > > Hm, I will fix it. Fixed. I'm attaching v5 patch-set. I've addressed review comments received so far and fixed a compiler warning that CF bot complained about. Please review it further. -- Bharath Rupireddy RDS Open Source Databases: https://aws.amazon.com/rds/postgresql/
v3-0001-Move-pg_pwritev_with_retry-to-file_utils.c.patch
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v3-0002-Use-pg_pwritev_with_retry-instead-of-write-in-wal.patch
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