[Devel] Re: [RFC][PATCH 1/7] Resource counters

[Herbert Poetzl]

On Tue, Mar 13, 2007 at 06:41:05PM +0300, Pavel Emelianov wrote:
> >>> PS: atomic_add_unless() didn't exist back then
> >>> (at least I think so) but that might be an option
> >>> too ...
> >> I think as far as having this discussion if you can remove that race
> >> people will be more willing to talk about what vserver does.
> > 
> > well, shouldn't be a big deal to brush that patch up
> > (if somebody actually _is_ interested)
> > 
> >> That said anything that uses locks or atomic operations (finer grained
> >> locks) because of the cache line ping pong is going to have scaling
> >> issues on large boxes.
> > 
> > right, but atomic ops have much less impact on most
> > architectures than locks :)
> 
> Right. But atomic_add_unless() is slower as it is
> essentially a loop. See my previous letter in this sub-thread.

fine, nobody actually uses atomic_add_unless(), or am I
missing something?

using two locks will be slower than using a single
lock, adding a loop which counts from 0 to 100 will
eat up some cpu, so what? don't do it :)

> >> So in that sense anything short of per cpu variables sucks at scale.
> >> That said I would much rather get a simple correct version without the
> >> complexity of per cpu counters, before we optimize the counters that
> >> much.
> > 
> > actually I thought about per cpu counters quite a lot, and
> > we (Llinux-VServer) use them for accounting, but please
> > tell me how you use per cpu structures for implementing 
> > limits
> 
> Did you ever look at how get_empty_filp() works?
> I agree, that this is not a "strict" limit, but it
> limits the usage wit some "precision".
> 
> /* off-the-topic */ Herbert, you've lost Balbir again:
> In this sub-thread some letters up Eric wrote a letter with
> Balbir in Cc:. The next reply from you doesn't include him.

I can happily add him to every email I reply to, but he
definitely isn't removed by my mailer (as I already stated,
it might be the mailing list which does this), fact is, the
email arrives here without him in the cc, so a reply does
not contain it either ...

best,
Herbert

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