fixup_pack_header_footer(): use nicely aligned buffer sizes

It should be more efficient to use nicely aligned buffer sizes, either
for filesystem operations or SHA1 checksums.  Also, using a relatively
small nominal size might allow for the data to remain in L1 cache
between both SHA1_Update() calls.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/pack-write.c b/pack-write.c
index f9776c5..939ed56 100644 (file)
--- a/pack-write.c
+++ b/pack-write.c
@@ -167,7 +167,7 @@ void fixup_pack_header_footer(int pack_fd,
                         unsigned char *partial_pack_sha1,
                         off_t partial_pack_offset)
 {
-       static const int buf_sz = 128 * 1024;
+       int aligned_sz, buf_sz = 8 * 1024;
        SHA_CTX old_sha1_ctx, new_sha1_ctx;
        struct pack_header hdr;
        char *buf;
@@ -188,10 +188,11 @@ void fixup_pack_header_footer(int pack_fd,
        partial_pack_offset -= sizeof(hdr);
 
        buf = xmalloc(buf_sz);
+       aligned_sz = buf_sz - sizeof(hdr);
        for (;;) {
                ssize_t m, n;
-               m = (partial_pack_sha1 && partial_pack_offset < buf_sz) ?
-                       partial_pack_offset : buf_sz;
+               m = (partial_pack_sha1 && partial_pack_offset < aligned_sz) ?
+                       partial_pack_offset : aligned_sz;
                n = xread(pack_fd, buf, m);
                if (!n)
                        break;
@@ -199,6 +200,10 @@ void fixup_pack_header_footer(int pack_fd,
                        die("Failed to checksum %s: %s", pack_name, strerror(errno));
                SHA1_Update(&new_sha1_ctx, buf, n);
 
+               aligned_sz -= n;
+               if (!aligned_sz)
+                       aligned_sz = buf_sz;
+
                if (!partial_pack_sha1)
                        continue;