Re: [PATCH] mm readahead: Fix sys_readahead breakage by reverting 2MB limit (bug 79111)

[Raghavendra K T]

On 07/04/2014 12:23 AM, Linus Torvalds wrote:
> On Thu, Jul 3, 2014 at 11:38 AM, Raghavendra K T
> <raghavendra.kt@xxxxxxxxxxxxxxxxxx> wrote:
> > Okay, how about something like 256MB? I would be happy to send a patch
> > for that change.
>
> I'd like to see some performance numbers. I know at least Fedora uses
> "readahead()" in the startup scripts, do we have any performance
> numbers for that?
>
> Also, I think 256MB is actually excessive. People still do have really
> slow devices out there. USB-2 is still common, and drives that read at
> 15MB/s are not unusual. Do we really want to do readahead() that can
> take tens of seconds (and *will* take tens of seconds sycnhronously,
> because the IO requests fill up).
>
> So I wouldn't go from 2 to 256. That seems like an excessive jump. I
> was more thinking in the 4-8MB range. But even then, I think we should
> always have technical reasons (ie preferably numbers) for the change,
> not just randomly change it.

Okay. I 'll take some time to do the analysis. I think we also should
keep in mind of possible remote readahead that would cause unnecessary
penalty.





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