Re: Index/hash order

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@osdl.org> wrote:

> > I've run a few tests, just to get a few numbers of the overhead
> > involved.  I used the last ~8,000 changesets from the BKCVS kernel
> > repository.  With cold cache, a checkout from cold cache takes about
> > 250 seconds on my laptop.  I don't have git numbers, but a mere copy
> > of the kernel tree needs 40 seconds.
> 
> I will bet you that a git checkout is _faster_ than a kernel source 
> tree copy. The time will be dominated by the IO costs (in particular 
> the read costs), and the IO costs are lower thanks to compression. So 
> I think that the cold-cache case will beat your 40 seconds by a clear 
> margin. It generally compresses to half the size, so 20 seconds is not 
> impossible (although seek costs would tend to stay constant, so I'd 
> expect it to be somewhere in between the two).

i'd be surprised if it was twice as fast - cache-cold linear checkouts 
are _seek_ limited, and it doesnt matter whether after a 1-2 msec 
track-to-track disk seek the DMA engine spends another 30 microseconds 
DMA-ing 60K uncompressed data instead of 30K compressed... (there are 
other factors, but this is the main thing.)

	Ingo