Re: ext2/3 performance regression in 2.6 vs 2.4 for small interl

[Giuliano Pochini]

On 12-Feb-2004 Andrea Arcangeli wrote:

> the main difference is that 2.4 isn't in function of time, it's in
> function of requests, no matter how long it takes to write a request,
> so it's potentially optimizing slow devices when you don't care about
> latency (deadline can be tuned for each dev via
> /sys/block/*/queue/iosched/).

IMHO it's the opposite. Transfer speed * seek time of some
slow devices is lower than fast devices. For example:

Hard disk  raw speed= 40MB/s   seek time =  8ms
MO/ZIP     raw speed=  3MB/s   seek time = 25ms

One seek of HD costs about 320KB, while on a slow drive it's
only 75KB. 2.4 has a terrible latency on slow devices, and it
has very small advantage in terms of speed. On CDs and DVDs
the cost of a seek is much higher, but since the data is
usually accessed sequentially you have the high latency
penalty with no appreciable speed gain in this case too.


--
Giuliano.
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