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SubjectRe: RFT: updatedb "morning after" problem [was: Re: -mm merge plans for 2.6.23]
On 07/28/2007 10:55 AM, wrote:

> 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.

Oh yes they are. Daniel for example did twice, telling me to turn my brain
on in between (if you read it, you may have noticed I got a little annoyed
at that point).

> 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 is. Prefetched pages can be dropped on the floor without additional I/O.

> 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)

Not to sound pretentious or anything but I assume that Andrew has a fairly
good overview of exactly how broken -mm can be at times. How many -mm users
use it anyway? He himself said he's not convinced of usefulness having not
seen it help for him (and notice that most developers are also users),
turned it off due to it annoying him at some point and hasn't seen a serious
investigation into potential downsides.

> that the only significant con left is the potential to mask other
> problems.

Which is not a madeup issue, mind you. As an example, I just now tried GNU
locate and saw it's a complete pig and specifically unsuitable for the low
memory boxes under discussion. Upon completion, it actually frees enough
memory that swap-prefetch _could_ help on some boxes, while the real issue
is that they should first and foremost dump GNU locate.

> 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)

I certainly would not want to argue anything of the sort no. As said a few
times, I agree that swap-prefetch makes sense and has at least the potential
to help some situations that you really wouldnt even want to try and fix any
other way, simply because nothing's broken.

> 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"

Well, _that_ is what the kernel is already going to great lengths at doing,
and it decided that those pages us poor overnight OO.o users want in in the
morning weren't reasonable guesses. The kernel also won't any time soon be
reading our minds, so any solution would need either user intervention (we
could devise a way to tell the kernel "hey ho, I consider these pages to be
very important -- try not to swap them out" possible even with a "and if you
do, please pull them back in when possible") or we can let swap-prefetch do
the "just in case" thing it is doing.

While swap-prefetch may not be the be all end all of solutions I agree that
having a machine sit around with free memory and applications in swap seems
not too useful if (as is the case) fetched pages can be dropped immediately
when it turns out swap-prefetch made the wrong decision.

So that's for the concept. As to implementation, if I try and look at the
code, it seems to be trying hard to really be free and as such, potential
downsides seem limited. It's a rather core concept though and as such needs
someone with a _lot_ more VM clue to ack. Sorry for not knowing, but who's
maintaining/submitting the thing now that Con's not? He or she should
preferably address any concerns it seems.


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