Re: Detrimental performance impact of ringbuffers on performance

On Tue, Apr 12, 2016 at 11:38 AM, Andres Freund <andres@anarazel.de> wrote:


>
>> The bottom line
>> here, IMHO, is not that there's anything wrong with our ring buffer
>> implementation, but that if you run PostgreSQL on a system where the
>> I/O is hitting a 5.25" floppy (not to say 8") the performance may be
>> less than ideal.  I really appreciate IBM donating hydra - it's been
>> invaluable over the years for improving PostgreSQL performance - but I
>> sure wish they had donated a better I/O subsystem.

When I had this problem some years ago, I traced it down to the fact
you have to sync the WAL before you can evict a dirty page.  If your
vacuum is doing a meaningful amount of cleaning, you encounter a dirty
page with a not-already-synced LSN about once per trip around the ring
buffer.   That really destroys your vacuuming performance with a 256kB
ring if your fsync actually has to reach spinning disk.  What I ended
up doing is hacking it so that it used a BAS_BULKWRITE when the vacuum
was being run with a zero vacuum cost delay.

> It's really not just hydra. I've seen the same problem on 24 disk raid-0
> type installations. The small ringbuffer leads to reads/writes being
> constantly interspersed, apparently defeating readahead.

Was their a BBU on that?  I would think slow fsyncs are more likely
than defeated readahead.  On the other hand, I don't hear about too
many 24-disk RAIDS without a BBU.