> Next question is what to do about it. I don't suppose we have any way
> of turning off the OS' read-ahead algorithm :-(. We could forget about
> this space-recycling improvement and go back to separate temp files.
> The objection to that, of course, is that while sorting might be faster,
> it doesn't matter how fast the algorithm is if you don't have the disk
> space to execute it.
Look what I found. I downloaded Linux kernel source for 2.2.0, and
started looking for the word 'ahead' in the file system files. I found
that read-ahead seems to be controlled by f_reada, and look where I
found it being turned off? Seems like any seek turns off read-ahead on
Linux.
When you do a read or write, it seems to be turned on again. Once you
read/write, the next read/write will do read-ahead, assuming you don't
do any lseek() before the second read/write().
Seems like the algorithm in psort now is rarely having read-ahead on
Linux, while other OS's check to see if the read-ahead was eventually
used, and control read-ahead that way.
read-head also seems be off on the first read from a file.
---------------------------------------------------------------------------
/** linux/fs/ext2/file.c
...
/** Make sure the offset never goes beyond the 32-bit mark..*/
static long long ext2_file_lseek(struct file *file,long long offset,int origin)
{struct inode *inode = file->f_dentry->d_inode;
switch (origin) { case 2: offset += inode->i_size; break; case 1: offset += file->f_pos;}if
(((unsignedlong long) offset >> 32) != 0) {
#if BITS_PER_LONG < 64 return -EINVAL;
#else if (offset > ext2_max_sizes[EXT2_BLOCK_SIZE_BITS(inode->i_sb)]) return -EINVAL;
#endif} if (offset != file->f_pos) { file->f_pos = offset; file->f_reada = 0; file->f_version =
++event;}returnoffset;
}
-- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610)
853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill,
Pennsylvania19026
