From: René Scharfe Date: Sat, 15 Apr 2017 17:55:52 GMT Subject: Re: [PATCH v10 3/3] read-cache: speed up add_index_entry during checkout Message-ID: In-Reply-To: <20170414191230.52825-4-git@jeffhostetler.com> Am 14.04.2017 um 21:12 schrieb git@jeffhostetler.com: > From: Jeff Hostetler > > Teach add_index_entry_with_check() and has_dir_name() > to see if the path of the new item is greater than the > last path in the index array before attempting to search > for it. > > During checkout, merge_working_tree() populates the new > index in sorted order, so this change will save at least 2 > binary lookups per file. This preserves the original > behavior but simply checks the last element before starting > the search. > > This helps performance on very large repositories. > > This can be seen using p0006-read-tree-checkout.sh and the > artificial repository created by t/perf/repos/many-files.sh > with parameters (5, 10, 9). (1M files in index.) > > Test HEAD^ HEAD > ---------------------------------------------------------------------------------------------------------- > 0006.2: read-tree br_base br_ballast (1000001) 4.15(2.72+1.41) 3.21(1.97+1.22) -22.7% > 0006.3: switch between br_base br_ballast (1000001) 8.11(5.57+2.28) 6.77(4.36+2.14) -16.5% > 0006.4: switch between br_ballast br_ballast_plus_1 (1000001) 13.52(8.68+4.35) 11.80(7.38+3.96) -12.7% > 0006.5: switch between aliases (1000001) 13.59(8.75+4.43) 11.85(7.49+3.87) -12.8% > > On linux.git, results are: > Test HEAD^ HEAD > ------------------------------------------------------------------------------------------------------ > 0006.2: read-tree br_base br_ballast (57994) 0.24(0.22+0.01) 0.20(0.17+0.02) -16.7% > 0006.3: switch between br_base br_ballast (57994) 9.91(5.82+2.79) 10.26(5.92+2.77) +3.5% > 0006.4: switch between br_ballast br_ballast_plus_1 (57994) 0.59(0.44+0.16) 0.50(0.34+0.18) -15.3% > 0006.5: switch between aliases (57994) 0.62(0.48+0.16) 0.50(0.35+0.16) -19.4% > > Signed-off-by: Jeff Hostetler > --- > read-cache.c | 118 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 116 insertions(+), 2 deletions(-) Very nice, especially the perf test! But can we unbundle the different optimizations into separate patches with their own performance numbers? Candidates IMHO: The change in add_index_entry_with_check(), the first hunk in has_dir_name(), the loop shortcuts. That might also help find the reason for the slight slowdown of 0006.3 for the kernel repository. > diff --git a/read-cache.c b/read-cache.c > index 97f13a1..ba95fbb 100644 > --- a/read-cache.c > +++ b/read-cache.c > @@ -910,6 +910,9 @@ int strcmp_offset(const char *s1, const char *s2, size_t *first_change) > /* > * Do we have another file with a pathname that is a proper > * subset of the name we're trying to add? > + * > + * That is, is there another file in the index with a path > + * that matches a sub-directory in the given entry? > */ > static int has_dir_name(struct index_state *istate, > const struct cache_entry *ce, int pos, int ok_to_replace) > @@ -918,9 +921,51 @@ static int has_dir_name(struct index_state *istate, > int stage = ce_stage(ce); > const char *name = ce->name; > const char *slash = name + ce_namelen(ce); > + size_t len_eq_last; > + int cmp_last = 0; > + > + /* > + * We are frequently called during an iteration on a sorted > + * list of pathnames and while building a new index. Therefore, > + * there is a high probability that this entry will eventually > + * be appended to the index, rather than inserted in the middle. > + * If we can confirm that, we can avoid binary searches on the > + * components of the pathname. > + * > + * Compare the entry's full path with the last path in the index. > + */ > + if (istate->cache_nr > 0) { > + cmp_last = strcmp_offset(name, > + istate->cache[istate->cache_nr - 1]->name, > + &len_eq_last); > + if (cmp_last > 0) { > + if (len_eq_last == 0) { > + /* > + * The entry sorts AFTER the last one in the > + * index and their paths have no common prefix, > + * so there cannot be a F/D conflict. > + */ > + return retval; > + } else { > + /* > + * The entry sorts AFTER the last one in the > + * index, but has a common prefix. Fall through > + * to the loop below to disect the entry's path > + * and see where the difference is. > + */ > + } > + } else if (cmp_last == 0) { > + /* > + * The entry exactly matches the last one in the > + * index, but because of multiple stage and CE_REMOVE > + * items, we fall through and let the regular search > + * code handle it. > + */ > + } > + } > > for (;;) { > - int len; > + size_t len; > > for (;;) { > if (*--slash == '/') > @@ -930,6 +975,66 @@ static int has_dir_name(struct index_state *istate, > } > len = slash - name; > > + if (cmp_last > 0) { > + /* > + * (len + 1) is a directory boundary (including > + * the trailing slash). And since the loop is > + * decrementing "slash", the first iteration is > + * the longest directory prefix; subsequent > + * iterations consider parent directories. > + */ > + > + if (len + 1 <= len_eq_last) { > + /* > + * The directory prefix (including the trailing > + * slash) also appears as a prefix in the last > + * entry, so the remainder cannot collide (because > + * strcmp said the whole path was greater). > + * > + * EQ: last: xxx/A > + * this: xxx/B > + * > + * LT: last: xxx/file_A > + * this: xxx/file_B > + */ > + return retval; > + } > + > + if (len > len_eq_last) { > + /* > + * This part of the directory prefix (excluding > + * the trailing slash) is longer than the known > + * equal portions, so this sub-directory cannot > + * collide with a file. > + * > + * GT: last: xxxA > + * this: xxxB/file > + */ > + return retval; > + } > + At this point len and len_eq_last are equal -- otherwise one of the two previous conditions would have triggered. Silly question: Is this necessary for the following shortcut to work? Removing the two checks above doesn't seem to affect performance very much, and at least the test suite still passes.. > + if (ce_namelen(istate->cache[istate->cache_nr - 1]) > len) { > + /* > + * The directory prefix lines up with part of > + * a longer file or directory name, but sorts > + * after it, so this sub-directory cannot > + * collide with a file. > + * > + * last: xxx/yy-file (because '-' sorts before '/') > + * this: xxx/yy/abc > + */ > + return retval; > + } istate->cache_nr can be zero if remove_index_entry_at() had been called in a previous iteration, resulting in a segfault. Checking right here is probably the easiest way out; not sure if exiting early when the index becomes empty would be better. > + > + /* > + * This is a possible collision. Fall through and > + * let the regular search code handle it. > + * > + * last: xxx > + * this: xxx/file > + */ > + } > + > pos = index_name_stage_pos(istate, name, len, stage); > if (pos >= 0) { > /* > @@ -1021,7 +1126,16 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e > > if (!(option & ADD_CACHE_KEEP_CACHE_TREE)) > cache_tree_invalidate_path(istate, ce->name); > - pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce)); > + > + /* > + * If this entry's path sorts after the last entry in the index, > + * we can avoid searching for it. > + */ > + if (istate->cache_nr > 0 && > + strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0) > + pos = -istate->cache_nr - 1; > + else > + pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce)); > > /* existing match? Just replace it. */ > if (pos >= 0) { >