Re: speed difference between using hard-linked and modular drives?

Anton Blanchard
2001-11-10T05:24:48Z

Hi,

> It _IS_ a big deal. Fetching _ONE_ hash chain cache line
> is always going to be cheaper than fetching _FIVE_ to _TEN_
> page struct cache lines while walking the list.

Exactly, the reason I found the pagecache hash was too small was because
__find_page_nolock was one of the worst offenders when doing zero copy
web serving of a large dataset.

> Even if prefetch would kill all of this overhead (sorry, it won't), it
> is _DUMB_ and _STUPID_ to bring those _FIVE_ to _TEN_ cache lines into
> the processor just to lookup _ONE_ page.

Yes you cant expect prefetch to help you when you use the data 10
instructions after you issue the prefetch. (ie walking the hash chain)

Anton