Messages in this thread | | | Date | Sat, 28 Jan 2006 01:35:03 +0100 | From | Eric Dumazet <> | Subject | Re: [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated |
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Eric Dumazet a écrit : > 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); > }
atomic_long_add(res, &fbc->count); res = atomic_long_read(&fbc->count);
> return res; > } > EXPORT_SYMBOL(percpu_counter_read_accurate); > #endif /* CONFIG_SMP */ >
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