Re: [PATCH] speed up SATA

From: Jeff Garzik
Date: Sun Mar 28 2004 - 12:50:55 EST

Jens Axboe wrote:
On Sun, Mar 28 2004, Jeff Garzik wrote:

Jens Axboe wrote:

On Sat, Mar 27 2004, Jeff Garzik wrote:

I also wouldn't want to lock out any users who wanted to use SATA at full speed ;-)

And full speed requires 32MB requests?

Full speed is the SATA driver supporting the hardware maximum. The

Come on Jeff, don't be such a slave to the hardware specifications. Just
because it's possible to send down 32MB requests doesn't necessarily
mean it's a super thing to do, nor that it automagically makes 'things
go faster'. The claim is that back-to-back 1MB requests are every bit as
fast as a 32MB request (especially if you have a small queue depth, in
that case there truly should be zero benefit to doing the bigger ones).
The cut-off point is likely even lower than 1MB, I'm just using that
figure as a value that is 'pretty big' yet doesn't incur too large
latencies just because of its size.

For me this is a policy issue.

I agree that huge requst hurt latency. I just disagree that the _driver_ should artificially lower its maximums to fit a guess about what the best request size should be.

If there needs to be an overall limit on per-size size, do it at the block layer. It's not scalable to hardcode that limit into every driver. That's not the driver's job. The driver just exports the hardware limits, nothing more.

A limit is fine. I support that. An artificial limit in the driver is not.


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