Re: -b vs. -n

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Jan 29 2009, Alan D. Brunelle wrote:
> Has anybody experimented with increasing the _number_ of buffers rather
> than the _size_ of the buffers when confronted with drops? I'm finding
> on a large(ish) system that it is better to have lots of small buffers
> handled by relay rather than fewer larger buffers. In my particular case:
> 
> 16 CPUs
> 96 devices
> running some dd's against all the devices...
> 
> -b 1024 or -b 2048 still results in drops
> 
> but:
> 
> -n 512 -b 16 allows things to run smoother.
> 
> I _think_ this may have to do with the way relay reports POLLIN: it does
> it only when a buffer switch happens as opposed to when there is data
> ready. Need to look at this some more, but just wondering if others out
> there have found similar things in their testing...

That's interesting. The reason why I exposed both parameters was mainly
that I didn't have the equipment to do adequate testing on what would be
the best setup for this. So perhaps we can change the README to reflect
that it is usually better to bump the number of buffers instead of the
size, if you run into overflow problems?

-- 
Jens Axboe