On Sun, 17 May 2009, david@xxxxxxx wrote:
>
> do things change with SSDs? I've heard that even (especially??) with the Intel
> SSDs you want to have several operations going in paralllel to get the best
> out of them.
There's a slight, but noticeable, improvement.
This is: "echo 3 > /proc/sys/vm/drop_caches; time git diff" run in a loop.
With 'core.preloadindex = true':
real 0m1.138s
real 0m1.116s
real 0m1.132s
real 0m1.120s
real 0m1.106s
real 0m1.132s
and with it set to 'false':
real 0m1.256s
real 0m1.258s
real 0m1.242s
real 0m1.240s
real 0m1.244s
real 0m1.242s
so it's about a 10% improvement. Which is pretty good, considering
that
(a) those disks are fast enough that even for that totally cache-cold
case, I get about 35% CPU utilization for the single-threaded case.
And that's despite this being a 3.2GHz Nehalem box, so 35% CPU is
really quite remarkably good. Om my (much slower) laptop with a
1.2GHz Core 2, I get 2-3% CPU-time (and the whole operation takes 20
seconds).
(b) Not all the IO ends up being parallelized, since there is a
per-directory mutex that means that even though we start 20 threads,
it probably gets a much smaller amount of real parallelism due to
locking.
in general, the IO parallelization obviously helps most when the IO is
slow _and_ overlaps perfectly. Perfect overlap doesn't end up happening
due to the per-directory lookup semaphore (think of it like a bank
conflict in trying to parallelize memory accesses), but with a slow NFS
connection you should get reasonably close to that optimal situation.
But with a single spindle, and rotating media, there really is sadly very
little room for optimization. I suspect a SATA with TCQ disk might be able
to do _somewhat_ better than my old PATA-only laptop (discounting the fact
that my PATA laptop harddisk is extra slow due to being just 4200rpm: any
desktop disk will be much faster), but I doubt the index preloading is
really all that noticeable.
In fact, I just tested on another machine, and saw no difference
what-so-ever. If anything, it was slightly slower. I suspect TCQ is a
bigger win with writes.
Linus
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