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Date: Mon, 13 Feb 2017 21:03:41 +0800 From: Wei Yang <richard.weiyang@...il.com> To: Tejun Heo <tj@...nel.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [RFC PATCH 2/2] mm/sparse: add last_section_nr in sparse_init() to reduce some iteration cycle On Sat, Feb 11, 2017 at 10:24 AM, Tejun Heo <tj@...nel.org> wrote: > > Hello, > Hi, Tejun Sorry for the delay, my gmail client seems to facing some problem. I can't see latest mails. So I have to use the web client and reply. > On Sat, Feb 11, 2017 at 10:18:29AM +0800, Wei Yang wrote: > > During the sparse_init(), it iterate on each possible section. On x86_64, > > it would always be (2^19) even there is not much memory. For example, on a > > typical 4G machine, it has only (2^5) to (2^6) present sections. This > > benefits more on a system with smaller memory. > > > > This patch calculates the last section number from the highest pfn and use > > this as the boundary of iteration. > > * How much does this actually matter? Can you measure the impact? > Hmm, I tried to print the "jiffies", while it is not ready at that moment. So I mimic the behavior in user space. I used following code for test. #include <stdio.h> #include <stdlib.h> int array[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; int main() { unsigned long i; int val; for (i = 0; i < (1UL << 5); i++) val += array[i%10]; for (i = 0; i < (1UL << 5); i++) val += array[i%10]; for (i = 0; i < (1UL << 5); i++) val += array[i%10]; //printf("%lx %d\n", i, val); return 0; } And compare the ruling with the iteration for the loop to be (1UL << 5) and (1UL << 19). The runtime is 0.00s and 0.04s respectively. The absolute value is not much. > * Do we really need to add full reverse iterator to just get the > highest section number? > You are right. After I sent out the mail, I realized just highest pfn is necessary. > Thanks. > > -- > tejun
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