Messages in this thread | | | From | Wei Yang <> | Date | Mon, 13 Feb 2017 21:03:41 +0800 | 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@kernel.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
| |