Re: Zerocopy NBD

[Steve Whitehouse <steve@gw.chygwyn.com>]

Hi,

> 
> On Wed, 30 May 2001, Steve Whitehouse wrote:
> >
[info about NBD patch deleted] 
> >
> Cool. 
> 
> Are you seeing performance improvements with the patch ?
>  

Yes, but my testing is not in anyway complete yet. The only network device
I have which is supported by zerocopy is loopback and there appear to be
problems with deadlocks when using NBD over loopback. So what I did was to
modify the NBD server (the userland one from Pavel Machek's web site)
so that it didn't actually do any disk I/O. It still copied the data from
the network into a buffer on write and it returns zeroed buffers on read
(not that thats important as only the write patch is affected in the patch).

I could then test using dd which is a bit artificial in that it creates
large requests giving probably much more data per NBD request than would
be usual under a filesystem load and hence also better with the zerocopy
patch. A timed dd with 100000 blocks of 1k spent 1.2 secs of system time
to do the write with NBD in 2.4.5 and 0.8 secs with my patch.

Also it may well be possible to adjust the network stack's memory management
to give better performance. I upped the values in tcp_[r|w]mem but I've
not checked what different vaules would do to those figures.

I want to do some more testing though in case I've made an error somewhere
in the method. I'd be particularly interested to hear from someone who
has any results for real hardware. If I have time I'll look into whether
the eepro100 or SysKonnect GigE cards could be made to support zerocopy
as they are the ones I have here,

Steve.