Below is oprofile output from GIT command 'git chekcout -q my-v2.6.25' (move from tag v2.6.27 to tag v2.6.25 of the Linux kernel):
CPU: Core 2, speed 1999.95 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit
mask of 0x00 (Unhalted core cycles) count 20000
Counted INST_RETIRED_ANY_P events (number of instructions retired) with a
unit mask of 0x00 (No unit mask) count 20000
CPU_CLK_UNHALT...|INST_RETIRED:2...|
samples| %| samples| %|
------------------------------------
409247 100.000 342878 100.000 git
CPU_CLK_UNHALT...|INST_RETIRED:2...|
samples| %| samples| %|
------------------------------------
260476 63.6476 257843 75.1996 libz.so.1.2.3
100876 24.6492 64378 18.7758 kernel-2.6.28.4_2.vmlinux
30850 7.5382 7874 2.2964 libc-2.9.so
14775 3.6103 8390 2.4469 git
2020 0.4936 4325 1.2614 libcrypto.so.0.9.8
191 0.0467 32 0.0093 libpthread-2.9.so
58 0.0142 36 0.0105 ld-2.9.so
1 2.4e-04 0 0 libldap-2.3.so.0.2.31Detail list of the top 20 function entries (libz counted in one blob):
CPU_CLK_UNHALTED INST_RETIRED_ANY_P samples % samples % image name symbol name 260476 63.6862 257843 75.2725 libz.so.1.2.3 /lib/libz.so.1.2.3 16587 4.0555 3636 1.0615 libc-2.9.so memcpy 7710 1.8851 277 0.0809 libc-2.9.so memmove 3679 0.8995 1108 0.3235 kernel-2.6.28.4_2.vmlinux d_validate 3546 0.8670 2607 0.7611 kernel-2.6.28.4_2.vmlinux __getblk 3174 0.7760 1813 0.5293 libc-2.9.so _int_malloc 2396 0.5858 3681 1.0746 kernel-2.6.28.4_2.vmlinux copy_to_user 2270 0.5550 2528 0.7380 kernel-2.6.28.4_2.vmlinux __link_path_walk 2205 0.5391 1797 0.5246 kernel-2.6.28.4_2.vmlinux ext4_mark_iloc_dirty 2103 0.5142 1203 0.3512 kernel-2.6.28.4_2.vmlinux find_first_zero_bit 2077 0.5078 997 0.2911 kernel-2.6.28.4_2.vmlinux do_get_write_access 2070 0.5061 514 0.1501 git cache_name_compare 2043 0.4995 1501 0.4382 kernel-2.6.28.4_2.vmlinux rcu_irq_exit 2022 0.4944 1732 0.5056 kernel-2.6.28.4_2.vmlinux __ext4_get_inode_loc 2020 0.4939 4325 1.2626 libcrypto.so.0.9.8 /usr/lib/libcrypto.so.0.9.8 1965 0.4804 1384 0.4040 git patch_delta 1708 0.4176 984 0.2873 kernel-2.6.28.4_2.vmlinux rcu_sched_grace_period 1682 0.4112 727 0.2122 kernel-2.6.28.4_2.vmlinux sysfs_slab_alias 1659 0.4056 290 0.0847 git find_pack_entry_one 1480 0.3619 1307 0.3816 kernel-2.6.28.4_2.vmlinux ext4_writepage_trans_blocks
Notice the memmove line, where the CPU did 7710 / 277 = 27.8 cycles per instruction, and compared to the total cycles spent inside the source code of GIT for this command, all the memmove() calls translates to (7710 * 100) / 14775 = 52.2% of this.
Retesting with a GIT program compiled for gcov usage, I found out that the memmove() calls came from remove_index_entry_at() in read-cache.c, where we have:
memmove(istate->cache + pos,
istate->cache + pos + 1,
(istate->cache_nr - pos) * sizeof(struct cache_entry *));remove_index_entry_at() is called 4902 times from check_updates() in unpack-trees.c, and each time called we move each cache_entry pointers (from the removed one) one step to the left.
Since we have 28828 entries in the cache this time, and if we on average move half of them each time, we in total move approximately 4902 * 0.5 * 28828 * 4 = 282 629 712 bytes, or twice this amount if each pointer is 8 bytes (64 bit).
OK, is seems that the function check_updates() is called 28 times, so the estimated guess above had been more correct if check_updates() had been called only once, but the point is: we get lots of bytes moved.
To fix this, and use an O(N) algorithm instead, where N is the number of cache_entries, we delete/remove all entries in one loop through all entries.
>From a retest, the new remove_marked_cache_entries() from the patch
below, ended up with the following output line from oprofile:
46 0.0105 15 0.0041 git remove_marked_cache_entries
If we can trust the numbers from oprofile in this case, we saved approximately ((7710 - 46) * 20000) / (2 * 1000 * 1000 * 1000) = 0.077 seconds CPU time with this fix for this particular test. And notice that now the CPU did only 46 / 15 = 3.1 cycles/instruction.
Signed-off-by: Kjetil Barvik <barvik@broadpark.no> --- cache.h | 1 + read-cache.c | 20 ++++++++++++++++++++ unpack-trees.c | 4 +--- 3 files changed, 22 insertions(+), 3 deletions(-)
Show changes to 3 files +22 −3
cache.h, read-cache.c, unpack-trees.c
diff --git a/cache.h b/cache.h index 6141269..0b11e3e 100644 --- a/cache.h +++ b/cache.h @@ -445,6 +445,7 @@ extern int add_index_entry(struct index_state *, struct cache_entry *ce, int opt extern struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int really); extern void rename_index_entry_at(struct index_state *, int pos, const char *new_name); extern int remove_index_entry_at(struct index_state *, int pos); +extern void remove_marked_cache_entries(struct index_state *istate); extern int remove_file_from_index(struct index_state *, const char *path); #define ADD_CACHE_VERBOSE 1 #define ADD_CACHE_PRETEND 2 diff --git a/read-cache.c b/read-cache.c index ca4bec2..bb07371 100644 --- a/read-cache.c +++ b/read-cache.c @@ -465,6 +465,26 @@ int remove_index_entry_at(struct index_state *istate, int pos) return 1; } +/* + * Remove all cache ententries marked for removal, that is where + * CE_REMOVE is set in ce_flags. This is much more effective than + * calling remove_index_entry_at() for each entry to be removed. + */ +void remove_marked_cache_entries(struct index_state *istate) +{ + struct cache_entry **ce_array = istate->cache; + unsigned int i, j; + + for (i = j = 0; i < istate->cache_nr; i++) { + if (ce_array[i]->ce_flags & CE_REMOVE) + remove_name_hash(ce_array[i]); + else + ce_array[j++] = ce_array[i]; + } + istate->cache_changed = 1; + istate->cache_nr = j; +} + int remove_file_from_index(struct index_state *istate, const char *path) { int pos = index_name_pos(istate, path, strlen(path)); diff --git a/unpack-trees.c b/unpack-trees.c index e3c3fa1..273b5da 100644 --- a/unpack-trees.c +++ b/unpack-trees.c @@ -93,11 +93,9 @@ static int check_updates(struct unpack_trees_options *o) display_progress(progress, ++cnt); if (o->update) unlink_entry(ce); - remove_index_entry_at(&o->result, i); - i--; - continue; } } + remove_marked_cache_entries(&o->result); remove_scheduled_dirs(); for (i = 0; i < index->cache_nr; i++) {
-- 1.6.1.349.g99fa5
