tree 23cce772ee2ef011410fd4cbbd2c418e9f312d71
parent 9ad2d5ea71d1470c2fd68b76c36b2ced3612cde3
author Derrick Stolee <dstolee@microsoft.com> 1617109864 +0000
committer Junio C Hamano <gitster@pobox.com> 1617134269 -0700

p2000: add sparse-index repos

p2000-sparse-operations.sh compares different Git commands in
repositories with many files at HEAD but using sparse-checkout to focus
on a small portion of those files.

Add extra copies of the repository that use the sparse-index format so
we can track how that affects the performance of different commands.

At this point in time, the sparse-index is 100% overhead from the CPU
front, and this is measurable in these tests:

Test
---------------------------------------------------------------
2000.2: git status (full-index-v3)              0.59(0.51+0.12)
2000.3: git status (full-index-v4)              0.59(0.52+0.11)
2000.4: git status (sparse-index-v3)            1.40(1.32+0.12)
2000.5: git status (sparse-index-v4)            1.41(1.36+0.08)
2000.6: git add -A (full-index-v3)              2.32(1.97+0.19)
2000.7: git add -A (full-index-v4)              2.17(1.92+0.14)
2000.8: git add -A (sparse-index-v3)            2.31(2.21+0.15)
2000.9: git add -A (sparse-index-v4)            2.30(2.20+0.13)
2000.10: git add . (full-index-v3)              2.39(2.02+0.20)
2000.11: git add . (full-index-v4)              2.20(1.94+0.16)
2000.12: git add . (sparse-index-v3)            2.36(2.27+0.12)
2000.13: git add . (sparse-index-v4)            2.33(2.21+0.16)
2000.14: git commit -a -m A (full-index-v3)     2.47(2.12+0.20)
2000.15: git commit -a -m A (full-index-v4)     2.26(2.00+0.17)
2000.16: git commit -a -m A (sparse-index-v3)   3.01(2.92+0.16)
2000.17: git commit -a -m A (sparse-index-v4)   3.01(2.94+0.15)

Note that there is very little difference between the v3 and v4 index
formats when the sparse-index is enabled. This is primarily due to the
fact that the relative file sizes are the same, and the command time is
mostly taken up by parsing tree objects to expand the sparse index into
a full one.

With the current file layout, the index file sizes are given by this
table:

       |  full index | sparse index |
       +-------------+--------------+
    v3 |     108 MiB |      1.6 MiB |
    v4 |      80 MiB |      1.2 MiB |

Future updates will improve the performance of Git commands when the
index is sparse.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>