| commit | 068fa54c0034be0b953d798628b06815f9cfaff0 | [log] [tgz] |
|---|---|---|
| author | Derrick Stolee <derrickstolee@github.com> | Tue Jul 19 15:26:06 2022 +0000 |
| committer | Junio C Hamano <gitster@pobox.com> | Tue Jul 19 08:38:17 2022 -0700 |
| tree | 0c150de0ca27ce1bba559829e887f942e221e17e | |
| parent | 90b2bb710da47f12967a6f6a32640c197ca82685 [diff] |
midx: reduce memory pressure while writing bitmaps
We noticed that some 'git multi-pack-index write --bitmap' processes
were running with very high memory. It turns out that a lot of this
memory is required to store a list of every object in the written
multi-pack-index, with a second copy that has additional information
used for the bitmap writing logic.
Using 'valgrind --tool=massif' before this change, the following chart
shows how memory load increased and was maintained throughout the
process:
GB
4.102^ ::
| @ @::@@::@@::::::::@::::::@@:#:::::::::::::@@:: :
| :::::@@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :::: :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :::: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| : :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| : :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| @ :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| @ :: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| @::: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| @::: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| @::: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
| @::: :: : :: @@:@: @ ::@ ::: ::::@: ::: @@:#:::::: :: : :@ :: :
0 +--------------------------------------------------------------->
It turns out that the 'struct write_midx_context' data is persisting
through the life of the process, including the 'entries' array. This
array is used last inside find_commits_for_midx_bitmap() within
write_midx_bitmap(). If we free (and nullify) the array at that point,
we can free a decent chunk of memory before the bitmap logic adds more
to the memory footprint.
Here is the massif memory load chart after this change:
GB
3.111^ #
| # :::::::::::@::::::::::::::@
| # ::::::::::::::::::::::::: : :: : @:: ::::: :: ::@
| @# :::::::::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| :::@#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| :: @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| :: @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
| :: @#::: ::: :::::: :::: :: : :::::::: : :: : @:: ::::: :: ::@
0 +--------------------------------------------------------------->
The previous change introduced a refactoring of write_midx_bitmap() to
make it more clear how much of the 'struct write_midx_context' instance
is needed at different parts of the process. In addition, the following
defensive programming measures were put in place:
1. Using FREE_AND_NULL() we will at least get a segfault from reading a
NULL pointer instead of a use-after-free.
2. 'entries_nr' is also set to zero to make any loop that would iterate
over the entries be trivial.
3. Add significant comments in write_midx_internal() to add warnings
for future authors who might accidentally add references to this
cleared memory.
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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