Обсуждение: Why the size (PQ_BUFFER_SIZE) of backend send buffers is 8192 ?

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Il 03/01/2014 09:47, xbzhang ha scritto:
> Why the size (PQ_BUFFER_SIZE) of backend send buffers  is 8192 ?
> Can i set it to other value ?

In principle, it is just an arbitrary amount by which to increase the
I/O buffer size. Of course it has to take into account the buffering
behaviour of the system kernel and TCP stack.

> If i extend or decrease it ,   can it affect the performance of
> sending message?

PQ_BUFFER_SIZE handle the low-level details of communication between
frontend and backend. I would not decrease its value, there is no
reason to expect advantage splitting 8k buffers (i.e. a page content
size) in my opinion.

Giuseppe.
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Giuseppe Broccolo - 2ndQuadrant Italy
PostgreSQL Training, Services and Support
giuseppe.broccolo@2ndQuadrant.it | www.2ndQuadrant.it
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xbzhang <xbzhang@kingbase.com.cn> writes:
> Why the size (PQ_BUFFER_SIZE) of backend send buffers  is 8192 ?

Traditionally, at least, that was the size of pipe buffers in Unix
machines, so in principle this is the most optimal chunk size for
sending data across a Unix socket.  I have no idea though if that's
still true in kernels in common use today.  For TCP communication
it might be marginally better to find out the MTU size and use that;
but it's unclear that it's worth the trouble, or indeed that we can
know the end-to-end MTU size with any reliability.

            regards, tom lane

On Fri, Jan 03, 2014 at 10:00:42AM -0500, Tom Lane wrote:
> know the end-to-end MTU size with any reliability.

Well, you could try PMTU discovery, though I agree that it's not
great.  It also seems pretty low-level for something like the DBMS to
be doing.

A

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Andrew Sullivan
ajs@crankycanuck.ca