Bruce,
On 11/22/05 4:13 PM, "Bruce Momjian" <pgman@xxxxxxxxxxxxxxxx> wrote:
> Perfect summary. We have a background writer now. Ideally we would
> have a background reader, that reads-ahead blocks into the buffer cache.
> The problem is that while there is a relatively long time between a
> buffer being dirtied and the time it must be on disk (checkpoint time),
> the read-ahead time is much shorter, requiring some kind of quick
> "create a thread" approach that could easily bog us down as outlined
> above.
Yes, the question is "how much read-ahead buffer is needed to equate to the
38% of I/O wait time in the current executor profile?"
The idea of asynchronous buffering would seem appropriate if the executor
would use the 38% of time as useful work.
A background reader is an interesting approach - it would require admin
management of buffers where AIO would leave that in the kernel. The
advantage over AIO would be more universal platform support I suppose?
> Right now the file system will do read-ahead for a heap scan (but not an
> index scan), but even then, there is time required to get that kernel
> block into the PostgreSQL shared buffers, backing up Luke's observation
> of heavy memcpy() usage.
As evidenced by the 16MB readahead setting still resulting in only 36% IO
wait.
> So what are our options? mmap()? I have no idea. Seems larger page
> size does help.
Not sure about that, we used to run with 32KB page size and I didn't see a
benefit on seq scan at all. I haven't seen tests in this thread that
compare 8K to 32K.
- Luke
