Issue #5995 has been updated by Yusuke Endoh.

Status changed from Assigned to Rejected

Hello,

There is indeed something wrong, but anyway, I agree with
kosaki's point; we cannot import the patch until we know
the exact reason why it brings performance improvement.
So please reopen this ticket if you find the reason.
(I expect kosaki-san to consider this!)


I wrote a simple C code to experiment this.  The result is
as kosaki said; when calling only one read syscall, posix_
fadvise does NOT bring performance improvement (even slower).
I really wonder why File#read becomes faster.



# using posix_fadvise
$ sudo sysctl -w vm.drop_caches=1 && time ./t dummy T
vm.drop_caches = 1
314572800

real    0m5.401s
user    0m0.000s
sys     0m0.740s


# NOT using posix_fadvise
$ sudo sysctl -w vm.drop_caches=1 && time ./t dummy F
vm.drop_caches = 1
314572800

real    0m3.967s
user    0m0.000s
sys     0m0.896s


#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>


int main(int argc, char *argv[])
{
    int fd = open(argv[1], O_RDONLY);
    char *buf;
    struct stat s;

    fstat(fd, &s);
    buf = malloc(s.st_size);

    if (argv[2][0] == 'T') {
        posix_fadvise(fd, 0, 0, POSIX_FADV_SEQUENTIAL);
    }

    printf("%d\n", read(fd, buf, s.st_size));

    return 0;
}

-- 
Yusuke Endoh <mame / tsg.ne.jp>
----------------------------------------
Feature #5995: calling io_advise_internal() in read_all()
https://bugs.ruby-lang.org/issues/5995

Author: Masaki Matsushita
Status: Rejected
Priority: Normal
Assignee: Motohiro KOSAKI
Category: core
Target version: 


=begin
I propose to call io_advise_internal() in read_all().
It will increase performance.

I created a dummy file:
 dd if=/dev/zero of=dummy bs=1M count=100

Then, I ran the following:

 require 'benchmark'
 
 Benchmark.bm do |x|
   x.report do
     f = File.open("dummy") # dummy file(about 100MB )
     f.read
   end
 end

I freed page cache before each test:
 sudo sysctl -w vm.drop_caches=1

results on Ubuntu 11.10(3.0.0-15-server):

r34462:

 user     system      total        real
   0.050000   0.220000   0.270000 (  0.356033)
 
       user     system      total        real
   0.050000   0.190000   0.240000 (  0.332243)

       user     system      total        real
   0.060000   0.210000   0.270000 (  0.347758)

patched ruby:

 user     system      total        real
   0.030000   0.130000   0.160000 (  0.225866)

       user     system      total        real
   0.040000   0.170000   0.210000 (  0.250172)

       user     system      total        real
   0.040000   0.150000   0.190000 (  0.254654)

It shows the patch increases performance.
=end


-- 
http://bugs.ruby-lang.org/