[Bug 222356] www/firefox: file-backed shared memory performance

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222356

--- Comment #4 from Tijl Coosemans <tijl at FreeBSD.org> ---
(In reply to Konstantin Belousov from comment #3)
It's UFS with softupdates and journaling.

This test program is more like what Firefox does.  It opens and unlinks a file,
sets the size with ftruncate and uses mmap.  There's no disk I/O that I can
notice except for the final close which causes a lot of I/O.  If you put munmap
last the I/O happens on that call.  The bigger the file the longer it takes. 
Surely this isn't all metadata?

#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int
main( void ) {
        int fd;
        size_t sz;
        void *base;

        sz = 128 * 1024 * 1024;
        fd = open( "nosync.data", O_RDWR | O_CREAT, 0600 );
        unlink( "nosync.data" );
        ftruncate( fd, sz );
        base = mmap( NULL, sz, PROT_READ | PROT_WRITE,
                     MAP_SHARED | MAP_NOSYNC, fd, 0 );
        puts( "calling memset" );
        memset( base, '0', sz );
        puts( "memset called" );
        sleep( 5 );
        puts( "calling munmap" );
        munmap( base, sz );
        puts( "munmap called" );
        sleep( 5 );
        puts( "calling close" );
        close( fd );
        puts( "close called" );
        sleep( 5 );
        return( 0 );
}


I didn't really investigate the problem with your patch.  I could build Firefox
fine and then suddenly gmake couldn't find some targets that were clearly
defined in the Makefile.  Rerunning the make command a couple times always gave
this same error.  Rebooting with the old kernel fixed it.

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