Messages in this thread | | | Date | Sat, 28 Jan 2006 01:28:20 +0100 | From | Eric Dumazet <> | Subject | Re: [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated |
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Andrew Morton a écrit : > Eric Dumazet <dada1@cosmosbay.com> wrote: >> Ravikiran G Thirumalai a écrit : >>> 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) ... >> Ah... yes you are right, I read min(16, NR_CPUS*2) > > So did I ;) > >> I wonder if it is not a typo... I mean, I understand the more cpus you have, >> the less updates on central atomic_t is desirable, but a quadratic offset >> seems too much... > > I'm not sure whether it was a mistake or if I intended it and didn't do the > sums on accuracy :( > > An advantage of retaining a spinlock in percpu_counter is that if accuracy > is needed at a low rate (say, /proc reading) we can take the lock and then > go spill each CPU's local count into the main one. It would need to be a > very low rate though. Or we make the cpu-local counters atomic too.
We might use atomic_long_t only (and no spinlocks) Something like this ?
struct percpu_counter { atomic_long_t count; atomic_long_t *counters; };
#ifdef CONFIG_SMP void percpu_counter_mod(struct percpu_counter *fbc, long amount) { long old, new; atomic_long_t *pcount;
pcount = per_cpu_ptr(fbc->counters, get_cpu()); start: old = atomic_long_read(pcount); new = old + amount; if (new >= FBC_BATCH || new <= -FBC_BATCH) { if (unlikely(atomic_long_cmpxchg(pcount, old, 0) != old)) goto start; atomic_long_add(new, &fbc->count); } else atomic_long_add(amount, pcount);
put_cpu(); } EXPORT_SYMBOL(percpu_counter_mod);
long percpu_counter_read_accurate(struct percpu_counter *fbc) { long res = 0; int cpu; atomic_long_t *pcount;
for_each_cpu(cpu) { pcount = per_cpu_ptr(fbc->counters, cpu); /* dont dirty cache line if not necessary */ if (atomic_long_read(pcount)) res += atomic_long_xchg(pcount, 0); } return res; } EXPORT_SYMBOL(percpu_counter_read_accurate); #endif /* CONFIG_SMP */
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