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SubjectRe: Zerocopy NBD

On Wed, 30 May 2001, Steve Whitehouse wrote:

> 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.

Copying bunchs of sequential data with 'dd' is OK for testing it ---
you're trying to measure only device speed, not fs speed.

> 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,


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