Re: [Aoetools-discuss] GGAoEd request merging [was:GGAoEd - initial evaluation feedback]

[Gabor G. <go...@sz...>]

On Fri, Jan 08, 2010 at 07:01:34PM +0200, Delian Krustev wrote:

> I just hoped that more requests would have been merged and thus played with:
> 
>   queue-length
>   max-delay
>   merge-delay
> 
> I was not able to get a better performance than with the default values though.
> 
> Why is not possible to have more requests merged ?
> E.g. what seems logical to me is to have bigger queue, increase the delays and
> have more requests merged ?

What ggaoed does is basically:

- read from the network until the kernel says "no more data"
- merge the requests if possible, and submit them in one go

>From your stats, on average the kernel queued about 3.5 packets by the
time ggaoed got woken up, and it indeed could merge those request almost
all of the time.

If you want more merging, you have to increase the number of requests
queued by the kernel. The queue length does not have a direct effect in
this, although it certainly limits the _maximum_ merging that can be
performed.

The 'merge-delay' parameter tells ggaoed not to start the I/O
immediately when there are no more incoming data available, but wait
for the specified time to allow receiving more packets and do more
merging. The 'merge-delay' however is directly added to the latency the
clients experience for a single request, so setting it too high will
also kill performance.

I consider 'merge-delay' experimental at this time as I do not know if
it really helps or not and I have no time to do extensive testing. But
if you want to play with it, provided you have a queue size of N,
measure/calculete how much time it takes for the client to send N/2
packets over the wire, and then set 'merge-delay' to this value. Then
you can increase/decrease it slightly to see if it has any effect.

The other option would be to tune your network driver to wait for more
incoming packets before notifying the operating system, if it has such a
capability.

Gabor

-- 
     ---------------------------------------------------------
     MTA SZTAKI Computer and Automation Research Institute
                Hungarian Academy of Sciences
     ---------------------------------------------------------