linux-kernel - Re: [RFC PATCH 2/2] mm/sparse: add last_section_nr in sparse

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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