Re: RFT: updatedb "morning after" problem [was: Re: -mm merge plans for 2.6.23]

[Daniel Hazelton <dhazelton@enter.net>]

On Saturday 28 July 2007 04:55:58 david@lang.hm wrote:
> On Sat, 28 Jul 2007, Rene Herman wrote:
> > On 07/27/2007 09:43 PM, david@lang.hm wrote:
> >>  On Fri, 27 Jul 2007, Rene Herman wrote:
> >> >  On 07/27/2007 07:45 PM, Daniel Hazelton wrote:
> >> > >   Questions about it:
> >> > >   Q) Does swap-prefetch help with this?
> >> > >   A) [From all reports I've seen (*)]
> >> > >   Yes, it does.
> >> >
> >> >  No it does not. If updatedb filled memory to the point of causing
> >> >  swapping (which noone is reproducing anyway) it HAS FILLED MEMORY and
> >> >  swap-prefetch hasn't any memory to prefetch into -- updatedb itself
> >> >  doesn't use any significant memory.
> >>
> >>  however there are other programs which are known to take up significant
> >>  amounts of memory and will cause the issue being described (openoffice
> >> for example)
> >>
> >>  please don't get hung up on the text 'updatedb' and accept that there
> >> are programs that do run intermittently and do use a significant amount
> >> of ram and then free it.
> >
> > Different issue. One that's worth pursueing perhaps, but a different
> > issue from the VFS caches issue that people have been trying to track
> > down.
>
> people are trying to track down the problem of their machine being slow
> until enough data is swapped back in to operate normally.
>
> in at some situations swap prefetch can help becouse something that used
> memory freed it so there is free memory that could be filled with data
> (which is something that Linux does agressivly in most other situations)
>
> in some other situations swap prefetch cannot help becouse useless data is
> getting cached at the expense of useful data.
>
> nobody is arguing that swap prefetch helps in the second cast.

Actually, I made a mistake when tracking the thread and reading the code for 
the patch and started to argue just that. But I have to admit I made a 
mistake - the patches author has stated (as Rene was kind enough to point 
out) that swap prefetch can't help when memory is filled.

> what people are arguing is that there are situations where it helps for
> the first case. on some machines and version of updatedb the nighly run of
> updatedb can cause both sets of problems. but the nightly updatedb run is
> not the only thing that can cause problems

Solving the cache filling memory case is difficult. There have been a number 
of discussions about it. The simplest solution, IMHO, would be to place a 
(configurable) hard limit on the maximum size any of the kernels caches can 
grow to. (The only solution that was discussed, however, is a complex beast)

>
> but let's talk about the concept here for a little bit
>
> the design is to use CPU and I/O capacity that's otherwise idle to fill
> free memory with data from swap.
>
> pro:
>    more ram has potentially useful data in it
>
> con:
>    it takes a little extra effort to give this memory to another app (the
> page must be removed from the list and zeroed at the time it's needed, I
> assume that the data is left in swap so that it doesn't have to be written
> out again)
>
>    it adds some complexity to the kernel (~500 lines IIRC from this thread)
>
>    by undoing recent swapouts it can potentially mask problems with swapout
>
> it looks to me like unless the code was really bad (and after 23 months in
> -mm it doesn't sound like it is) that the only significant con left is the
> potential to mask other problems.

I'll second this. But with the swap system itself having seen as heavy testing 
as it has I don't know if it would be masking other problems.

That is why I've been asking "What is so wrong with it?" - while it definately 
doesn't help with programs that cause caches to balloon (that problem does 
need another solution) it does help to speed things up when a memory hog has 
exited. (And since its a pretty safe assumption that swap is going to be 
noticeably slower than RAM this patch seems to me to be a rather visible and 
obvious solution to that problem)

> however there are many legitimate cases where it is definantly dong the
> right thing (swapout was correct in pushing out the pages, but now the
> cause of that preasure is gone). the amount of benifit from this will vary
> from situation to situation, but it's not reasonable to claim that this
> provides no benifit (you have benchmark numbers that show it in synthetic
> benchmarks, and you have user reports that show it in the real-worlk)

Exactly. Though I have seen posts which (to me at least) appear to claim 
exactly that. It was part of the reason why I got a bit incensed. (The other 
was that it looked like the kernel devs with the ultra-powerful machines were 
claiming 'I don't see the problem on my machine, so it doesn't exist'. That 
sort of attitude is fine, in some cases, but not, IMHO, where performance is 
concerned)

> there are lots of things in the kernel who's job is to pre-fill the memroy
> with data that may (or may not) be useful in the future. this is just
> another method of filling the cache. it does so my saying "the user wanted
> these pages in the recent past, so it's a reasonable guess to say that the
> user will want them again in the future"

Yep. And it's a pretty obvious step forward. The VFS system already does 
readahead and caching for mounted volumes to improve performance - why not do 
similar to improve the performance of swap?

The only real downside is that swap-prefetch won't be effective in all cases 
and it will cause some extra power consumption. (drives can't spin-down as 
soon as the would without it, etc...) While I can only make some suggestions 
as to how to fix the problem of ballooning caches (I've been wading through 
the VM code for a few days now and still don't fully understand any of it), 
the solution to the power consumption seems obvious - swap prefetch doesn't 
work when the system is running on battery (or UPS or whatever)

DRH

-- 
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