Date: Wed, 29 Sep 2010 11:40:05 -0400 From: David Cross <crossd@cs.rpi.edu> To: freebsd-hackers@freebsd.org Subject: Unnecessary reads on write load Message-ID: <4CA35DD5.7070900@cs.rpi.edu>
next in thread | raw e-mail | index | archive | help
Tracking down a performance issue where the system apparently needlessly
reads on a 100% write load... consider the following C test case: (more
after the code)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
int main(int argc, char **argv)
{
unsigned char dir1[4], dir2[4], filename[15], pathname[1024],
buffer[130944];
unsigned int filenum, count, filesize;
int fd;
arc4random_buf(buffer, 131072);
count=atoi(argv[1]);
for(;count>0;count--) {
filenum=arc4random();
filesize=arc4random_uniform(130944) + 128;
sprintf(filename, "%08x", filenum);
strncpy(dir1,filename,3);
strncpy(dir2,filename+3, 3);
dir1[3]=dir2[3]=0;
sprintf(pathname, "%s/%s/%s", dir1, dir2, filename);
fd=open(pathname, O_CREAT | O_WRONLY, 0644);
if (fd < 0) {
sprintf(pathname, "%s/%s", dir1, dir2);
if (mkdir(pathname, 0755) < 0) {
mkdir(dir1, 0755);
mkdir(pathname, 0755);
};
sprintf(pathname, "%s/%s/%s", dir1, dir2, filename);
fd=open(pathname, O_CREAT | O_WRONLY, 0644);
}
if (fd <0)
continue;
write(fd,buffer, filesize);
close(fd);
}
return 0;
}
In running that in an empty directory (it takes one argument, the number
of files to create), I see that it spends most of its time in BIORD?!.
If I have a debugging kernel I can see that its all in NAMI cache
misses, and doing block fetches... but "why?" the only directories that
exist are ones that it just created, and should therefore be in the
cache, right? Any ideas? Give it a try yourself and tell me what you get.
--
David E. Cross
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?4CA35DD5.7070900>