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@xxxxxxx>
---
builtin-pack-objects.c | 35 ++++++++++++++++++++++++++++++++++-
1 files changed, 34 insertions(+), 1 deletions(-)
diff --git a/builtin-pack-objects.c b/builtin-pack-objects.c
index 4319e7b..f5dcd20 100644
--- a/builtin-pack-objects.c
+++ b/builtin-pack-objects.c
@@ -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 +302,11 @@ 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 +319,11 @@ 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 +1457,29 @@ 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.
--
1.5.5.1.226.g6f6e8
--
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