Messages in this thread | | | Date | Fri, 27 Jan 2006 15:01:06 -0800 | From | Andrew Morton <> | Subject | Re: [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated |
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Ravikiran G Thirumalai <kiran@scalex86.org> wrote: > > On Fri, Jan 27, 2006 at 12:16:02PM -0800, Andrew Morton wrote: > > Ravikiran G Thirumalai <kiran@scalex86.org> wrote: > > > > > > which can be assumed as not frequent. > > > At sk_stream_mem_schedule(), read_sockets_allocated() is invoked only > > > certain conditions, under memory pressure -- on a large CPU count machine, > > > you'd have large memory, and I don't think read_sockets_allocated would get > > > called often. It did not atleast on our 8cpu/16G box. So this should be OK > > > I think. > > > > That being said, the percpu_counters aren't a terribly successful concept > > and probably do need a revisit due to the high inaccuracy at high CPU > > counts. It might be better to do some generic version of vm_acct_memory() > > instead. > > AFAICS vm_acct_memory is no better. The deviation on large cpu counts is the > same as percpu_counters -- (NR_CPUS * NR_CPUS * 2) ...
I suppose so. Except vm_acct_memory() has
#define ACCT_THRESHOLD max(16, NR_CPUS * 2)
But if we were to perform similar tuning to percpu_counter, yes, they're pretty similar.
Oh, and because vm_acct_memory() is counting a singleton object, it can use DEFINE_PER_CPU rather than alloc_percpu(), so it saves on a bit of kmalloc overhead.
> > > > If the benchmarks say that we need to. If we cannot observe any problems > > in testing of existing code and if we can't demonstrate any benefit from > > the patched code then one option is to go off and do something else ;) > > We first tried plain per-CPU counters for memory_allocated, found that reads > on memory_allocated was causing cacheline transfers, and then > switched over to batching. So batching reads is useful. To avoid > inaccuracy, we can maybe change percpu_counter_init to: > > void percpu_counter_init(struct percpu_counter *fbc, int maxdev) > > the percpu batching limit would then be maxdev/num_possible_cpus. One would > use batching counters only when both reads and writes are frequent. With > the above scheme, we would go fetch cachelines from other cpus for read > often only on large cpu counts, which is not any worse than the global > counter alternative, but it would still be beneficial on smaller machines, > without sacrificing a pre-set deviation. > > Comments?
Sounds sane. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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