On Tue, Mar 13, 2007 at 03:09:06AM -0600, Eric W. Biederman wrote: > Herbert Poetzl <[EMAIL PROTECTED]> writes: > > > On Sun, Mar 11, 2007 at 01:00:15PM -0600, Eric W. Biederman wrote: > >> Herbert Poetzl <[EMAIL PROTECTED]> writes: > >> > >> > > >> > Linux-VServer does the accounting with atomic counters, > >> > so that works quite fine, just do the checks at the > >> > beginning of whatever resource allocation and the > >> > accounting once the resource is acquired ... > >> > >> Atomic operations versus locks is only a granularity thing. > >> You still need the cache line which is the cost on SMP. > >> > >> Are you using atomic_add_return or atomic_add_unless or > >> are you performing you actions in two separate steps > >> which is racy? What I have seen indicates you are using > >> a racy two separate operation form. > > > > yes, this is the current implementation which > > is more than sufficient, but I'm aware of the > > potential issues here, and I have an experimental > > patch sitting here which removes this race with > > the following change: > > > > - doesn't store the accounted value but > > limit - accounted (i.e. the free resource) > > - uses atomic_add_return() > > - when negative, an error is returned and > > the resource amount is added back > > > > changes to the limit have to adjust the 'current' > > value too, but that is again simple and atomic > > > > best, > > Herbert > > > > 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 :) > 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 TIA, Herbert > Eric - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
