On Wed, Jul 10, 2013 at 4:55 AM, Jeff King <p...@peff.net> wrote: > The pack revindex stores the offsets of the objects in the > pack in sorted order, allowing us to easily find the on-disk > size of each object. To compute it, we populate an array > with the offsets from the sha1-sorted idx file, and then use > qsort to order it by offsets. > > That does O(n log n) offset comparisons, and profiling shows > that we spend most of our time in cmp_offset. However, since > we are sorting on a simple off_t, we can use numeric sorts > that perform better. A radix sort can run in O(k*n), where k > is the number of "digits" in our number. For a 64-bit off_t, > using 16-bit "digits" gives us k=4. > > On the linux.git repo, with about 3M objects to sort, this > yields a 400% speedup. Here are the best-of-five numbers for > running "echo HEAD | git cat-file --batch-disk-size", which > is dominated by time spent building the pack revindex: > > before after > real 0m0.834s 0m0.204s > user 0m0.788s 0m0.164s > sys 0m0.040s 0m0.036s > > On a smaller repo, the radix sort would not be > as impressive (and could even be worse), as we are trading > the log(n) factor for the k=4 of the radix sort. However, > even on git.git, with 173K objects, it shows some > improvement: > > before after > real 0m0.046s 0m0.017s > user 0m0.036s 0m0.012s > sys 0m0.008s 0m0.000s

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k should only be 2 for git.git. I haven't packed in a while, but I think it should all fit within 4G. :) The pathological case would be a pack file with very few very very large objects, large enough to push the pack size over the 2^48 threshold so we'd have to do all four radixes. It's probably worth mentioning here and/or in the code that k is dependent on the pack file size and that we can jump out early for small pack files. That's my favorite part of this code by the way. :) > Signed-off-by: Jeff King <p...@peff.net> > --- > I changed a few things from the original, including: > > 1. We take an "unsigned" number of objects to match the fix in the > last patch. > > 2. The 16-bit "digit" size is factored out to a single place, which > avoids magic numbers and repeating ourselves. > > 3. The "digits" variable is renamed to "bits", which is more accurate. > > 4. The outer loop condition uses the simpler "while (max >> bits)". > > 5. We use memcpy instead of an open-coded loop to copy the whole array > at the end. The individual bucket-assignment is still done by > struct assignment. I haven't timed if memcpy would make a > difference there. > > 6. The 64K*sizeof(int) "pos" array is now heap-allocated, in case > there are platforms with a small stack. > > I re-ran my timings to make sure none of the above impacted them; it > turned out the same. > > pack-revindex.c | 84 > +++++++++++++++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 79 insertions(+), 5 deletions(-) > > diff --git a/pack-revindex.c b/pack-revindex.c > index 1aa9754..9365bc2 100644 > --- a/pack-revindex.c > +++ b/pack-revindex.c > @@ -59,11 +59,85 @@ static int cmp_offset(const void *a_, const void *b_) > /* revindex elements are lazily initialized */ > } > > -static int cmp_offset(const void *a_, const void *b_) > +/* > + * This is a least-significant-digit radix sort. > + */ > +static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t > max) > { > - const struct revindex_entry *a = a_; > - const struct revindex_entry *b = b_; > - return (a->offset < b->offset) ? -1 : (a->offset > b->offset) ? 1 : 0; > + /* > + * We use a "digit" size of 16 bits. That keeps our memory > + * usage reasonable, and we can generally (for a 4G or smaller > + * packfile) quit after two rounds of radix-sorting. > + */ > +#define DIGIT_SIZE (16) > +#define BUCKETS (1 << DIGIT_SIZE) > + /* > + * We want to know the bucket that a[i] will go into when we are using > + * the digit that is N bits from the (least significant) end. > + */ > +#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1)) > + > + /* > + * We need O(n) temporary storage, so we sort back and forth between > + * the real array and our tmp storage. To keep them straight, we > always > + * sort from "a" into buckets in "b". > + */ > + struct revindex_entry *tmp = xcalloc(n, sizeof(*tmp)); Didn't notice it the first time I read this, but do we really need calloc here? Or will malloc do? > + struct revindex_entry *a = entries, *b = tmp; > + int bits = 0; > + unsigned *pos = xmalloc(BUCKETS * sizeof(*pos)); > + > + while (max >> bits) { > + struct revindex_entry *swap; > + int i; You forgot to make i unsigned. See below too... > + > + memset(pos, 0, BUCKETS * sizeof(*pos)); > + > + /* > + * We want pos[i] to store the index of the last element that > + * will go in bucket "i" (actually one past the last element). > + * To do this, we first count the items that will go in each > + * bucket, which gives us a relative offset from the last > + * bucket. We can then cumulatively add the index from the > + * previous bucket to get the true index. > + */ > + for (i = 0; i < n; i++) > + pos[BUCKET_FOR(a, i, bits)]++; > + for (i = 1; i < BUCKETS; i++) > + pos[i] += pos[i-1]; > + > + /* > + * Now we can drop the elements into their correct buckets (in > + * our temporary array). We iterate the pos counter backwards > + * to avoid using an extra index to count up. And since we are > + * going backwards there, we must also go backwards through > the > + * array itself, to keep the sort stable. > + */ > + for (i = n - 1; i >= 0; i--) > + b[--pos[BUCKET_FOR(a, i, bits)]] = a[i]; ...which is why the above loop still works. > + > + /* > + * Now "b" contains the most sorted list, so we swap "a" and > + * "b" for the next iteration. > + */ > + swap = a; > + a = b; > + b = swap; > + > + /* And bump our bits for the next round. */ > + bits += DIGIT_SIZE; > + } > + > + /* > + * If we ended with our data in the original array, great. If not, > + * we have to move it back from the temporary storage. > + */ > + if (a != entries) > + memcpy(entries, tmp, n * sizeof(*entries)); > + free(tmp); > + free(pos); > + > +#undef BUCKET_FOR > } > > /* > @@ -108,7 +182,7 @@ static void create_pack_revindex(struct pack_revindex > *rix) > */ > rix->revindex[num_ent].offset = p->pack_size - 20; > rix->revindex[num_ent].nr = -1; > - qsort(rix->revindex, num_ent, sizeof(*rix->revindex), cmp_offset); > + sort_revindex(rix->revindex, num_ent, p->pack_size); > } > > struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs) > -- > 1.8.3.rc3.24.gec82cb9 -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html