Re: Keepalive for max_standby_delay

On 17/05/10 04:40, Simon Riggs wrote:
> On Sun, 2010-05-16 at 16:53 +0100, Simon Riggs wrote:
>>>
>>> Attached patch rearranges the walsender loops slightly to fix the above.
>>> XLogSend() now only sends up to MAX_SEND_SIZE bytes (== XLOG_SEG_SIZE /
>>> 2) in one round and returns to the main loop after that even if there's
>>> unsent WAL, and the main loop no longer sleeps if there's unsent WAL.
>>> That way the main loop gets to respond to signals quickly, and we also
>>> get to update the shared memory status and PS display more often when
>>> there's a lot of catching up to do.
>>>
>>> Comments
>>
>> 8MB at a time still seems like a large batch to me.
>>
>> libpq is going to send it in smaller chunks anyway, so I don't see the
>> importance of trying to keep the batch too large. It just introduces
>> delay into the sending process. We should be sending chunks that matches
>> libpq better.
>
> More to the point the logic will fail if XLOG_BLCKSZ> PQ_BUFFER_SIZE
> because it will send partial pages.

I don't see a failure. We rely on not splitting WAL records across
messages, but we're talking about libpq-level CopyData messages, not TCP
messages.

> Having MAX_SEND_SIZE> PQ_BUFFER_SIZE is pointless, as libpq currently
> stands.

Well, it does affect the size of the read() in walsender, and I'm sure
there's some overhead in setting the ps display and the other small
stuff we do once per message. But you're probably right that we could
easily make MAX_SEND_SIZE much smaller with no noticeable affect on
performance, while making walsender more responsive to signals. I'll
decrease it to, say, 512 kB.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com