Re: 2.6.20-ck1

From: Con Kolivas
Date: Sat Feb 17 2007 - 19:40:07 EST


Andrew Morton writes:

On Sun, 18 Feb 2007 08:00:06 +1100 Con Kolivas <kernel@xxxxxxxxxxx> wrote:

On Sunday 18 February 2007 05:45, Chuck Ebbert wrote:
...
> But the one I like, mm-filesize_dependant_lru_cache_add.patch,
> has an on-off switch.
>

...

Do you still want this patch for mainline?...

Don't think so. The problems I see are:

- It's a system-wide knob. In many situations this will do the wrong
thing. Controlling pagecache should be per-process.

- Its heuristics for working out when to invalidate the pagecache will be
too much for some situations and too little for others.

- Whatever we do, there will be some applications in some situations
which are hurt badly by changes like this: they'll do heaps of extra IO.


Generally, the penalties for getting this stuff wrong are very very high:
orders of magnitude slowdowns in the right situations. Which I suspect
will make any system-wide knob ultimately unsuccessful.

Rest assured I wasn't interested in pushing this patch for mainline anyway.

-ck users can also rest assured about this patch for the following reasons:

- The usage pattern on a desktop will guarantee that this patch helps 99.9% of the time rather than hurts. Therefore, this feature is enabled by default on -ck.

- With the usage pattern on a server of any sort, it will be unknown whether this patch helps or harms. Therefore, this feature is disabled by default on -cks.

--
-ck

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