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Hans Reiser wrote:
> Guys, if you run the kernel under a debugger, and get it to where you
> see the excessive CPU usage, and then start stepping through the bitmap
> code, I am sure it will be very obvious what the error is. Can anyone
> do that for us? Jeff?
Apologies to everyone CC'd who've already seen this message. It was
bounced from the namesys servers and I wanted to preserve the CC list.
***
Mike sent me a copy of the metadata and I am now able to reproduce
locally. My profiling looks like this:
samples % image name app name symbol name
148596 17.8573 reiserfs.ko reiserfs reiserfs_in_journal
58194 6.9934 reiserfs.ko reiserfs search_by_key
38937 4.6792 vmlinux vmlinux memmove
38783 4.6607 reiserfs.ko reiserfs scan_bitmap_block
38466 4.6226 jbd jbd (no symbols)
23249 2.7939 vmlinux vmlinux __find_get_block
18196 2.1867 vmlinux vmlinux tty_write
17734 2.1312 vmlinux vmlinux do_ioctl
17293 2.0782 loop loop (no symbols)
15400 1.8507 vmlinux vmlinux cond_resched_lock
14836 1.7829 vmlinux vmlinux copy_user_generic_c
14143 1.6996 reiserfs.ko reiserfs do_journal_end
13638 1.6389 vmlinux vmlinux find_next_zero_bit
13236 1.5906 vmlinux vmlinux default_llseek
12925 1.5532 vmlinux vmlinux bit_waitqueue
8921 1.0721 vmlinux vmlinux __delay
Hans -
My speculation about the bitmaps being fragmented was right on. I wrote
a quick little script to parse the output of debugreiserfs -m and report
on the frequency of different window sizes. Windows of 1-31 blocks are
extremely common, accounting for 99.8% of all free windows. The problem
is that in my testing, where I made the allocator report the size of
allocation requests, the most common request was for a window of 32 blocks.
What's happening is that we keep finding windows that are too small,
which results in a lot of wasted effort. The cycle goes like this:
if (unfm && is_block_in_journal(s, bmap_n, *beg, beg))
continue;
/* first zero bit found; we check next bits */
for (end = *beg + 1;; end++) {
if (end >= *beg + max || end >= boundary
|| reiserfs_test_le_bit(end, bi->bh->b_data)) {
next = end;
break;
}
/* finding the other end of zero bit window requires
* looking into journal structures (in
* case of searching for free blocks for unformatted nodes) */
if (unfm && is_block_in_journal(s, bmap_n, end, &next))
break;
}
If the window is too small, we end up looping up to the top and try to
find another one. Since the overwhelming majority of the windows are too
small, we go through just about all the bitmaps without backing off the
window size.
- -Jeff
- --
Jeff Mahoney
SUSE Labs
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