| commit | 4c6c7971e0e42f0f1ce7e9472bc3aec8e0c96650 | [log] [tgz] |
|---|---|---|
| author | Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com> | Fri Jan 10 01:59:30 2020 +0000 |
| committer | Junio C Hamano <gitster@pobox.com> | Fri Jan 10 11:34:36 2020 -0800 |
| tree | 669d16fd615c46335e245fe613c40e20ceb2b3fc | |
| parent | 757ff352bd66de31715580f8c525e54ad30b22eb [diff] |
unpack-trees: correctly compute result count The clear_ce_flags_dir() method processes the cache entries within a common directory. The returned int is the number of cache entries processed by that directory. When using the sparse-checkout feature in cone mode, we can skip the pattern matching for entries in the directories that are entirely included or entirely excluded. eb42feca (unpack-trees: hash less in cone mode, 2019-11-21) introduced this performance feature. The old mechanism relied on the counts returned by calling clear_ce_flags_1(), but the new mechanism calculated the number of rows by subtracting "cache_end" from "cache" to find the size of the range. However, the equation is wrong because it divides by sizeof(struct cache_entry *). This is not how pointer arithmetic works! A coverity build of Git for Windows in preparation for the 2.25.0 release found this issue with the warning, "Pointer differences, such as cache_end - cache, are automatically scaled down by the size (8 bytes) of the pointed-to type (struct cache_entry *). Most likely, the division by sizeof(struct cache_entry *) is extraneous and should be eliminated." This warning is correct. This leaves us with the question "how did this even work?" The problem that occurs with this incorrect pointer arithmetic is a performance-only bug, and a very slight one at that. Since the entry count returned by clear_ce_flags_dir() is reduced by a factor of 8, the loop in clear_ce_flags_1() will re-process entries from those directories. By inserting global counters into unpack-tree.c and tracing them with trace2_data_intmax() (in a private change, for testing), I was able to see count how many times the loop inside clear_ce_flags_1() processed an entry and how many times clear_ce_flags_dir() was called. Each of these are reduced by at least a factor of 8 with the current change. A factor larger than 8 happens when multiple levels of directories are repeated. Specifically, in the Linux kernel repo, the command git sparse-checkout set LICENSES restricts the working directory to only the files at root and in the LICENSES directory. Here are the measured counts: clear_ce_flags_1 loop blocks: Before: 11,520 After: 1,621 clear_ce_flags_dir calls: Before: 7,048 After: 606 While these are dramatic counts, the time spent in clear_ce_flags_1() is under one millisecond in each case, so the improvement is not measurable as an end-to-end time. Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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