Re: [PATCH 2/2] Implement a simple delta_base cache

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sat, 17 Mar 2007, Linus Torvalds wrote:
> 
>  (a) it probably doesn't really matter a lot (but I don't have the 
>      numbers)

Well, to some degree I obviously *do* have the numbers.

I have the numbers that we used to re-generate the object data over five 
*hundred* times per object for some cases, and that I got the average 
such delta-base usage down from 20x to 1.1-1.3x depending on cache size.

In contrast, the "use delta-base also for non-delta queries" fairly 
obviously cannot touch those kinds of numbers. We migth avoid a *few* 
object generation cases, but we're not looking at factors of 20 for any 
kind of sane cases.

So I do think that a higher-level caching approach can work too, but it's 
going to be more effective in other areas:

 - get rid of some ugly hacks (like the "save_commit_buffer" thing I 
   mentioned)
 - possibly help some insane loads (eg cases where we really *do* end up 
   seeing the same object over and over again, perhaps simply because some 
   idiotic automated commit system ends up switching between a few states 
   back-and-forth).

I really think the "insane loads" thing is unlikely, but I could construct 
some crazy usage scenario where a cache of objects in general (and not 
just delta bases) would work. I don't think it's a very realistic case, 
but who knows - people sometimes do really stupid things.

		Linus