Messages in this thread | | | Date | Tue, 7 Mar 2006 19:08:03 -0800 | From | Ravikiran G Thirumalai <> | Subject | Re: [patch 2/4] net: percpufy frequently used vars -- struct proto.memory_allocated |
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On Tue, Mar 07, 2006 at 06:14:22PM -0800, Andrew Morton wrote: > Ravikiran G Thirumalai <kiran@scalex86.org> wrote: > > > > - if (atomic_read(sk->sk_prot->memory_allocated) < sk->sk_prot->sysctl_mem[0]) { > > + if (percpu_counter_read(sk->sk_prot->memory_allocated) < > > + sk->sk_prot->sysctl_mem[0]) { > > Bear in mind that percpu_counter_read[_positive] can be inaccurate on large > CPU counts. > > It might be worth running percpu_counter_sum() to get the exact count if we > think we're about to cause something to fail.
The problem is percpu_counter_sum has to read all the cpus cachelines. If we have to use percpu_counter_sum everywhere, then might as well use plain per-cpu counters instead of batching ones no?
sysctl_mem[0] is about 196K and on a 16 cpu box variance is 512 bytes, which is OK with just percpu_counter_read I hope. Maybe, on very large cpu counts, we should just change the FBC_BATCH so that variance does not go quadratic. Something like 32. So that variance is 32 * NR_CPUS in that case, instead of (NR_CPUS * NR_CPUS * 2) currently. Comments? - 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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