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refs: write packed_refs file using stdio

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We write each line of a new packed-refs file individually
using a write() syscall (and sometimes 2, if the ref is
peeled). Since each line is only about 50-100 bytes long,
this creates a lot of system call overhead.

We can instead open a stdio handle around our descriptor and
use fprintf to write to it. The extra buffering is not a
problem for us, because nobody will read our new packed-refs
file until we call commit_lock_file (by which point we have
flushed everything).

On a pathological repository with 8.5 million refs, this
dropped the time to run `git pack-refs` from 20s to 6s.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jeff King
2014-09-10 06:03:52 -04:00
committed by Junio C Hamano
parent 0c72b98f31
commit 9540ce5030
3 changed files with 33 additions and 23 deletions
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2
cache.h
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@ -1395,6 +1395,8 @@ extern const char *git_mailmap_blob;
/* IO helper functions */
extern void maybe_flush_or_die(FILE *, const char *);
__attribute__((format (printf, 2, 3)))
extern void fprintf_or_die(FILE *, const char *fmt, ...);
extern int copy_fd(int ifd, int ofd);
extern int copy_file(const char *dst, const char *src, int mode);
extern int copy_file_with_time(const char *dst, const char *src, int mode);