On Sun, Jul 7, 2013 at 3:14 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 > > Signed-off-by: Jeff King <p...@peff.net> > --- > I think there are probably still two potential issues here: > > 1. My while() loop termination probably has issues when we have to use > all 64 bits to represent the pack offset (not likely, but...) > > 2. We put "int pos[65536]" on the stack. This is a little big, but is > probably OK, as I think the usual small stack problems we have seen > are always in threaded code. But it would not be a big deal to heap > allocate it (it would happen once per radix step, which is only 4 > times for the whole sort). > > pack-revindex.c | 77 > +++++++++++++++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 72 insertions(+), 5 deletions(-) > > diff --git a/pack-revindex.c b/pack-revindex.c > index 77a0465..d2adf36 100644 > --- a/pack-revindex.c > +++ b/pack-revindex.c > @@ -59,11 +59,78 @@ 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 using a 16-bit "digit". > + */ > +static void sort_revindex(struct revindex_entry *entries, int n, off_t max)

If 'n' is the number of objects in the pack, shouldn't it be unsigned? The data type for struct packed_git.num_objects is uint32_t. Looks like create_pack_revindex uses the wrong datatype when it captures num_objects in the int num_ent and passes it to sort_revindex. So, it looks like that function needs to be updated too. > { > - 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 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)); > + struct revindex_entry *a = entries, *b = tmp; > + int digits = 0; > + > + /* > + * 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, digits) ((a[i].offset >> digits) & 0xffff) > + > + while (max / (((off_t)1) << digits)) { Is there any reason this shouldn't be simplified to just: while (max >> digits) { I glanced briefly at the assembly and it appears that gcc does actually emit a divide instruction to accomplish this, which I think we can avoid by just rearranging the operation. > + struct revindex_entry *swap; > + int i; > + int pos[65536] = {0}; > + > + /* > + * 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, digits)]++; > + for (i = 1; i < ARRAY_SIZE(pos); 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, digits)]] = a[i]; > + > + /* > + * 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 digits for the next round. */ > + digits += 16; > + } > + > + /* > + * 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) { > + int i; > + for (i = 0; i < n; i++) > + entries[i] = tmp[i]; I think I recall that somebody investigated whether a for loop like you have above was faster for copying structures than memcpy. I forget whether it was conclusive. Did you happen to compare them? <snip> -Brandon -- 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