Messages in this thread | | | From | Jörg-Volker Peetz <> | Subject | Re: [PATCH v2] proc: speedup /proc/stat handling | Date | Mon, 30 Jan 2012 11:00:58 +0100 |
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Eric Dumazet wrote, on 01/30/12 10:25: > Le lundi 30 janvier 2012 à 09:06 +0100, Jörg-Volker Peetz a écrit : >> Eric Dumazet wrote, on 01/25/12 01:26: >>> Le mercredi 25 janvier 2012 à 09:18 +0900, KAMEZAWA Hiroyuki a écrit : >>> >>>> BTW, what is the reason of this change ? >>>> >>>>> - unsigned size = 4096 * (1 + num_possible_cpus() / 32); >>>>> + unsigned size = 1024 + 128 * num_possible_cpus(); >>>> >>>> I think size of buffer is affected by the number of online cpus. >>>> (Maybe 128 is enough but please add comment why 128 ?) >>>> >>> >>> There is no change, as 4096/32 is 128 bytes per cpu. >>> >> >> Wrong math, only num_possible_cpus() is divided by 32. Thus, >> >> - unsigned size = 4096 * (1 + num_possible_cpus() / 32); >> + unsigned size = 4096 + 128 * num_possible_cpus(); >> >> <snip> > > > It is good math, once you take the time to think a bit about it. > > The original question was about the 128 * num_possible_cpus() > > 4096/32 is 128 as I said. > > The 4096 -> 1024 is just taking into account fact that once you do the > correct computations, you dont need initial 4096 value, and 1024 is more > than enough.
Thanks for your explanation, that makes sense now. At first, I was mislead by your comment "There is no change" which was apparently related to the second term in the formula.
> > Example on a dual core machine : > > # dmesg|grep nr_irq > [ 0.000000] nr_irqs_gsi: 40 > [ 0.000000] NR_IRQS:2304 nr_irqs:712 16 > > size = 1024 + 2*128 + 2*712 = 2704 bytes (rounded to 4096 by kmalloc()) > > # wc -c /proc/stat > 1767 /proc/stat > > Problem with original math was that for a machine with 16 cpus or a > machine with 1 cpu, we ended with the same 4096 value. That was a real > problem. > > If we instead use "unsigned size = 4096 + 128 * num_possible_cpus();" as > you suggest, we would always allocate 2 pages of memory, this is not > needed at all for typical 1/2/4 way machines. > -- Best regards, Jörg-Volker.
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