On Fri, 5 Oct 2001, Dan Nelson wrote:
> This rules out mysql as the cause for the delay.
I agree.
> > > I'd say start dumping packets on the network.
> >
> > I'd agree, but I'm confused as to why a different query (that
> > requests more data; 33 rows vs 1) can reliably execute and fetch in
> > 10ms on all machines? The behaviour is completely reproducible:
> > SELECT (1 record) ON PRIMARY KEY = slow (200ms), SELECT (lots of
> > records on indexed field) = fast (10ms)
>
> The fact that your two queries take different times to process has
> nothing to do with indices, and more to do with the bytecount of the
> query and the response.
>
> > > 200ms sounds a lot like Nagle's Algorithm kicking in (which
> > > shouldn't happen assuming the mysql libs are written right).
> >
> > Indeed, I wouldn't have thought they'd have included that! Isn't
> > Nagle restricted to telnet? But anyway, not all queries perform
> > equally badly.
>
> Nagle's algorithm applies to all TCP sessions unless explicitly
> disabled. It buffers outgoing data less than your MSS for up to 200ms
> if there is unacknowleged data already on the wire. This is usually
> triggered by inefficient code on the sending end that does multiple
> writes(); the first write() gets sent immediately. Any subsequent
> writes() get buffered up by Nagle until 200ms or the ACK for the first
> block of data from the receiving machine. The standard fix is to
> rewrite the sending code to send all its data in a single write(), but
> the simple fix (which ends up wasting bandwidth by sending many small
> packets) is to
>
> int var=1;
> setsockopt(socket, IPPROTO_TCP, TCP_NODELAY, &var, sizeof(var));
>
> Chances are your two windows machines have differnt myodbc versions, or
> different TCP settings in the registry, that make Nagle kick in at
> different times.
Great. That is exactly what I was trying to point out in my last post.
But done really poorly.
tt,
--
Boyd Gerber <gerberb@stripped>
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