as I just now was in the situation to call
update-grub quite a few times
and hit this bug, and found it somewhat annoying,
I tried to track it down.
# time update-grub
Searching for GRUB installation directory ... found: /boot/grub
Searching for default file ... found: /boot/grub/default
Testing for an existing GRUB menu.lst file ... found:
/boot/grub/menu.lst
Searching for splash image ... found: /boot/grub/splash.xpm.gz
Found kernel: ...
Updating /boot/grub/menu.lst ... done
real 1m18.955s
user 0m2.612s
sys 0m41.603s
80 seconds wall clock time.
doh.
(yes, I did that several times, and it is consistent)
my root is on LVM, btw.
strace and gdb revealed
that it is spending much time in repeatedly
doing open_device for my PV,
basically you get this access pattern:
open()
getgeom
flushbufs
lseek
read 4k
close
repeated about 32 thousand times ;)
I tracked this down
grub_lvm_scan_device,
disk/lvm.c:289 err = grub_disk_read (disk, 0, mda_offset, mda_size,
metadatabuf);
actually to this loop in grub_disk_read:
/* Until SIZE is zero... */
while (size)
{
char *data;
grub_disk_addr_t start_sector;
grub_size_t len;
grub_size_t pos;
/* For reading bulk data. */
start_sector = sector & ~(GRUB_DISK_CACHE_SIZE - 1);
pos = (sector - start_sector) << GRUB_DISK_SECTOR_BITS;
len = ((GRUB_DISK_SECTOR_SIZE << GRUB_DISK_CACHE_BITS)
- pos - real_offset);
if (len > size)
len = size;
/* Fetch the cache. */
data = grub_disk_cache_fetch (disk->dev->id, disk->id, start_sector);
if (data)
...
else
{
/* Otherwise read data from the disk actually. */
if ((disk->dev->read) (disk, start_sector,
GRUB_DISK_CACHE_SIZE, tmp_buf)
!= GRUB_ERR_NONE)
now, guess what GRUB_DISK_CACHE_SIZE is? right, 4k.
and, what happens to be my mda_size? hm, happens to be 128 MiB.
so I changed:
# cat debian/patches/00_increase_grub_disk_cache.diff
--- a/include/grub/disk.h.orig 2010-01-12 19:30:32.125277561 +0100
+++ b/include/grub/disk.h 2010-01-12 19:33:47.261239353 +0100
@@ -131,8 +131,8 @@
#define GRUB_DISK_CACHE_NUM 1021
/* The size of a disk cache in sector units. */
-#define GRUB_DISK_CACHE_SIZE 8
-#define GRUB_DISK_CACHE_BITS 3
+#define GRUB_DISK_CACHE_BITS 11
+#define GRUB_DISK_CACHE_SIZE (1<<GRUB_DISK_CACHE_BITS)
/* This is called from the memory manager. */
void grub_disk_cache_invalidate_all (void);
rebuilt, installed:
r...@rum:~/src# dpkg -i grub-common_1.96+20080724-16.1_amd64.deb
timed again:
# time update-grub
...
Updating /boot/grub/menu.lst ... done
real 0m13.463s
user 0m1.668s
sys 0m5.592s
not too bad for a "one line" patch.
comments welcome, maybe this approach is not the best, maybe 1MiB is overkill,
maybe I forgot to adjust some other "grub disk cache" internals...
maybe rather (or additionally) keep a single instance static last-used cache
inside of open_device(), which could save a lot of syscalls as well.
also, as has been mentioned before, some special mode for grub-probe
would be great so it need not be called five times from udate-grub.
what does grub upstream have to say about this?
Cheers,
Lars
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