On Thu, Mar 05, 2009, thestar@xxxxxxxxxxxxxxxx wrote:
> > The current git-p4 way of concatenating strings performs in O(n^2)
> >and is therefore terribly slow with large files because of unnecessary
> >memory copies. The following patch makes the operation O(n).
>
> The reason why it uses simple concatenation is to cut down on memory usage.
> - It is a tradeoff.
>
> I think the modification you have made below is reasonable, however be
> aware that memory usage could double, which substantially reduce the
> size of the changesets that git-p4 would be able to import /at all/,
> rather than to merely be slow.
Uhm, no. The memory usage could be an additional X, where X is the
size of the biggest file in the commit. Remember that commit() stores
the complete commit data in memory before sending it to fast-import.
Also, on my machine the extra memory is already used because at some
point, "text += foo" calls realloc() anyway and often duplicates the
memory used by text.
The ideal solution is to use a generator and refactor the commit
handling as a stream. I am working on that but it involves deeper
changes, so as I am not sure it will be accepted, I'm providing the
attached compromise patch first. At least it solves the appaling speed
issue. I tuned it so that it never uses more than 32 MiB extra memory.
Signed-off-by: Sam Hocevar <sam@xxxxxxx>
---
contrib/fast-import/git-p4 | 10 +++++++++-
1 files changed, 9 insertions(+), 1 deletions(-)
diff --git a/contrib/fast-import/git-p4 b/contrib/fast-import/git-p4
index 3832f60..151ae1c 100755
--- a/contrib/fast-import/git-p4
+++ b/contrib/fast-import/git-p4
@@ -984,11 +984,19 @@ class P4Sync(Command):
while j < len(filedata):
stat = filedata[j]
j += 1
+ data = []
text = ''
while j < len(filedata) and filedata[j]['code'] in ('text', 'unicod
e', 'binary'):
- text += filedata[j]['data']
+ data.append(filedata[j]['data'])
del filedata[j]['data']
+ # p4 sends 4k chunks, make sure we don't use more than 32 MiB
+ # of additional memory while rebuilding the file data.
+ if len(data) > 8192:
+ text += ''.join(data)
+ data = []
j += 1
+ text += ''.join(data)
+ del data
if not stat.has_key('depotFile'):
sys.stderr.write("p4 print fails with: %s\n" % repr(stat))
--
Sam.
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