* Jens Axboe <[email protected]> wrote:
> > Seems a pretty fundamental change which could do with some careful
> > benchmarking, methinks.
> >
> > See, your patch amounts to "do more seeks to improve one test case".
> > Surely other testcases will worsen. What are they?
>
> Yes, completely agree! I think Arjans patch makes a heap of sense, but
> some numbers would be great to see.
Arjan gave the relevant hard numbers:
| With latencytop, I noticed that the (in memory) atime updates during a
| kernel build had latencies of 600 msec or longer [...]
|
| With this patch, the latencies for atime updates (and similar
| operation) go down by a factor of 3x to 4x !
atime update latencies went down by a factor of 3x-4x ...
but what bothers me even more is the large picture. Linux's development
is still fundamentally skewed towards bandwidth (which goes up with
hardware advances anyway), while the focus on latencies is very lacking
(which users do care about much more and which usually does _not_
improve with improved hardware), so i cannot see why we shouldnt apply
this. Reminds me of the illogical, almost superstitious resistence
against the relatime patch. (which is not in 2.6.24 mind you - killed
for good)
if bandwidth hurts anywhere, it will be pointed out and fixed, we've got
like tons of bandwidth benchmarks and it's _easy_ to fix bandwidth
problems. But _finally_ we now have desktop latency tools, hard numbers
and patches that fix them, but what do we do ... we put up extra
roadblocks??
so lets just goddamn apply this _trivial_ patch. This isnt an intrusive
1000 line rewrite that is hard to revert. If it causes any bandwidth
problems, it will be just as trivial to undo. If we do anything else we
just stiffle the still young and very much under-represented "lets fix
latencies that bothers people" movement. If anything we need _positive_
discrimination for latency related fixes (which treatment this fix does
not need at all - all it needs is _equal_ footing with the countless
bandwidth patches that go into the kernel all the time), otherwise it
will never take off and become as healthy as bandwidth optimizations.
Ok?
Ingo
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