Re: [PATCH 10/10] pack-revindex: radix-sort the revindex

From: Ramkumar Ramachandra <artagnon@gmail.com>
Date: Jul 10, 2013, 13:17 UTC
Message-ID: <CALkWK0=_6-RN2dHrhMdKu8nTyj1iwVe388Y57pOac17xhbqn5Q@mail.gmail.com>
In-Reply-To: <20130710115557.GJ21963@sigill.intra.peff.net>

Jeff King wrote:

Show 6 quoted lines

> 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.

Wait, isn't off_t a signed data type? Did you account for that in your algorithm?

> 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:

Okay.

Show 12 quoted lines

> 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.
> + */

Any particular reason for choosing LSD, and not MSD?

> +#define DIGIT_SIZE (16)
> +#define BUCKETS (1 << DIGIT_SIZE)

Okay, NUMBER_OF_BUCKETS = 2^RADIX, and you choose a hex radix. Is off_t guaranteed to be fixed-length though? I thought only the ones in stdint.h were guaranteed to be fixed-length?

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> +       /*
> +        * 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))

Ouch! This is unreadable. Just write an inline function instead? A % would've been easier on the eyes, but you chose base-16.

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> +       /*
> +        * 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));

Shouldn't this be sizeof (struct revindex_entry), since tmp hasn't been declared yet? Also, s/n/revindex_nr/, and something more appropriate for tmp?

> +       struct revindex_entry *a = entries, *b = tmp;

It's starting to look like you have something against descriptive names ;)

> +       int bits = 0;
> +       unsigned *pos = xmalloc(BUCKETS * sizeof(*pos));

sizeof(unsigned int), for clarity, if not anything else. You picked malloc over calloc here, because you didn't want to incur the extra cost of zero-initializing the memory? Also, pos is the actual buckets array, I presume (hence unsigned, because there can't be a negative number of keys in any bucket)?

> +       while (max >> bits) {

No clue what max is. Looked at the caller and figured out that it's the pack-size, although I'm still clueless about why it's appearing here.

> +               struct revindex_entry *swap;
> +               int i;
> +
> +               memset(pos, 0, BUCKETS * sizeof(*pos));

Ah, so that's why you used malloc there. Wait, shouldn't this be memset(pos, 0, sizeof(*pos))?

> +               for (i = 0; i < n; i++)
> +                       pos[BUCKET_FOR(a, i, bits)]++;

Okay, so you know how many numbers are in each bucket.

> +               for (i = 1; i < BUCKETS; i++)
> +                       pos[i] += pos[i-1];

Cumulative sums; right.

> +               for (i = n - 1; i >= 0; i--)
> +                       b[--pos[BUCKET_FOR(a, i, bits)]] = a[i];

Classical queue. You could've gone for something more complex, but I don't think it would have been worth the extra complexity.

> +               swap = a;
> +               a = b;
> +               b = swap;

Wait a minute: why don't you just throw away b? You're going to rebuild the queue in the next iteration anyway, no? a is what is being sorted.

> +               /* And bump our bits for the next round. */
> +               bits += DIGIT_SIZE;

I'd have gone for a nice for-loop.

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> +       /*
> +        * 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));

How could a be different from entries? It has no memory allocated for itself, no? Why did you even create a, and not directly operate on entries?

> +       free(tmp);
> +       free(pos);

Overall, I found it quite confusing :(

> +#undef BUCKET_FOR

Why not DIGIT_SIZE and BUCKETS too, while at it?