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. > > 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.
BTW atomic_add_unless() is essentially a loop!!! Just like spin_lock() is, so why is one better that another? spin_lock() can go to schedule() on preemptive kernels thus increasing interactivity, while atomic can't. > 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. > > 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/
