Messages in this thread | | | Date | Thu, 24 Apr 2008 17:04:26 +0200 | From | Jens Axboe <> | Subject | Re: [RFC][PATCH 0/3] Skip I/O merges when disabled |
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On Thu, Apr 24 2008, Andi Kleen wrote: > > > Not a good idea IMHO, it's much better with an explicit setting. That > > way you don't introduce indeterministic behavior. > > So you would be deterministically slower.
Yes, absolutely. Think about the case for a second - the potential gain is in fractions of a percent basically, the potential loss however is HUGE. There's absolutely no way on earth I'd ever make this dynamic.
> Another way to avoid this problem would be to keep the statistics per > IO context, then the same run of a program would always get the same > behaviour. Drawback is that if your non mergeable workload consists of > lots of short running processes (like a shell script) the optimization > wouldn't work. Not sure if it's really practical, but it would be an option.
Complexity for basically zero gain, no thanks.
> I think in modern systems with caches etc. you typically have enough > non quite deterministic and other surprising and hard to analyze > behaviour anyways, so a little more doesn't make much difference.
Sorry Andi, but that is nonsense. Not merging IOs when you should can cut your performance to a 5th or something of that order, it's an entirely different ballgame.
-- Jens Axboe
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