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pack-objects: allow for early delta deflating

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When the delta data is cached in memory until it is written to a pack
file on disk, it is best to compress it right away in find_deltas() for
the following reasons:

  - we have to compress that data anyway;

  - this allows for caching more deltas with the same cache size limit;

  - compression is potentially threaded.

This last point is especially relevant for SMP run time.  For example,
repacking the Linux repo on a quad core processor using 4 threads with
all default settings produce the following results before this change:

	real    2m27.929s
	user    4m36.492s
	sys     0m3.091s

And with this change applied:

	real    2m13.787s
	user    4m37.486s
	sys     0m3.159s

So the actual execution time stayed more or less the same but the
wall clock time is shorter.

This is however not a good thing to do when generating a pack for
network transmission.  In that case, the network is most likely to
throttle the data throughput, so it is best to make find_deltas()
faster in order to start writing data ASAP since we can afford
spending more time between writes to compress the data
at that point.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Nicolas Pitre 2008-05-02 15:11:50 -04:00 committed by Junio C Hamano
parent 30ebb40aa1
commit ed4a9031ea
1 changed files with 36 additions and 1 deletions
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37
builtin-pack-objects.c
View file

@ -43,6 +43,7 @@ struct object_entry {
*/
void *delta_data; /* cached delta (uncompressed) */
unsigned long delta_size; /* delta data size (uncompressed) */
unsigned long z_delta_size; /* delta data size (compressed) */
unsigned int hash; /* name hint hash */
enum object_type type;
enum object_type in_pack_type; /* could be delta */
@ -301,6 +302,13 @@ static unsigned long write_object(struct sha1file *f,
buf = read_sha1_file(entry->idx.sha1, &type, &size);
if (!buf)
die("unable to read %s", sha1_to_hex(entry->idx.sha1));
/*
* make sure no cached delta data remains from a
* previous attempt before a pack split occured.
*/
free(entry->delta_data);
entry->delta_data = NULL;
entry->z_delta_size = 0;
} else if (entry->delta_data) {
size = entry->delta_size;
buf = entry->delta_data;
@ -313,7 +321,11 @@ static unsigned long write_object(struct sha1file *f,
type = (allow_ofs_delta && entry->delta->idx.offset) ?
OBJ_OFS_DELTA : OBJ_REF_DELTA;
}
datalen = do_compress(&buf, size);
if (entry->z_delta_size)
datalen = entry->z_delta_size;
else
datalen = do_compress(&buf, size);
/*
* The object header is a byte of 'type' followed by zero or
@ -1447,6 +1459,29 @@ static void find_deltas(struct object_entry **list, unsigned *list_size,
best_base = other_idx;
}
/*
* If we decided to cache the delta data, then it is best
* to compress it right away. First because we have to do
* it anyway, and doing it here while we're threaded will
* save a lot of time in the non threaded write phase,
* as well as allow for caching more deltas within
* the same cache size limit.
* ...
* But only if not writing to stdout, since in that case
* the network is most likely throttling writes anyway,
* and therefore it is best to go to the write phase ASAP
* instead, as we can afford spending more time compressing
* between writes at that moment.
*/
if (entry->delta_data && !pack_to_stdout) {
entry->z_delta_size = do_compress(&entry->delta_data,
entry->delta_size);
cache_lock();
delta_cache_size -= entry->delta_size;
delta_cache_size += entry->z_delta_size;
cache_unlock();
}
/* if we made n a delta, and if n is already at max
* depth, leaving it in the window is pointless. we
* should evict it first.