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Commit8002e8ee18(builtin/cat-file: use bitmaps to efficiently filter by object type, 2025-04-02) introduced a performance regression when we are not filtering objects: it uses bitmaps even when they won't help, incurring extra costs. For example, running the new perf tests from this commit, which check the performance of listing objects by oid: $ export GIT_PERF_LARGE_REPO=/path/to/linux.git $ git -C "$GIT_PERF_LARGE_REPO" repack -adb $ GIT_SKIP_TESTS=p1006.1 ./run 8002e8ee18^8002e8ee18p1006-cat-file.sh [...] Test 8002e8ee18^8002e8ee18------------------------------------------------------------------------------- 1006.2: list all objects (sorted) 1.48(1.44+0.04) 6.39(6.35+0.04) +331.8% 1006.3: list all objects (unsorted) 3.01(2.97+0.04) 3.40(3.29+0.10) +13.0% 1006.4: list blobs 4.85(4.67+0.17) 1.68(1.58+0.10) -65.4% An invocation that filters, like listing all blobs (1006.4), does benefit from using the bitmaps; it now doesn't have to check the type of each object from the pack data, so the tradeoff is worth it. But for listing all objects in sorted idx order (1006.2), we otherwise would never open the bitmap nor the revindex file. Worse, our sorting step gets much worse. Normally we append into an array in pack .idx order, and the sort step is trivial. But with bitmaps, we get the objects in pack order, which is apparently random with respect to oid, and have to sort the whole thing. (Note that this freshly-packed state represents the best case for .idx sorting; if we had two packs, then we'd have their objects one after the other and qsort would have to interleave them). The unsorted test in 1006.3 is interesting: there we are going in pack order, so we load the revindex for the pack anyway. And though we don't sort the result, we do use an oidset to check for duplicates. So we can see in the 8002e8ee18^ timings that those two things cost ~1.5s over the sorted case (mostly the oidset hash cost). We also incur the extra cost to open the bitmap file as of8002e8ee18, which seems to be ~400ms. (This would probably be faster with a bitmap lookup table, but writing that out is not yet the default). So we know that bitmaps help when there's filtering to be done, but otherwise make things worse. Let's only use them when there's a filter. The perf script shows that we've fixed the regressions without hurting the bitmap case: Test 8002e8ee18^8002e8ee18HEAD -------------------------------------------------------------------------------------------------------- 1006.2: list all objects (sorted) 1.56(1.53+0.03) 6.44(6.37+0.06) +312.8% 1.62(1.54+0.06) +3.8% 1006.3: list all objects (unsorted) 3.04(2.98+0.06) 3.45(3.38+0.07) +13.5% 3.04(2.99+0.04) +0.0% 1006.4: list blobs 5.14(4.98+0.15) 1.76(1.68+0.06) -65.8% 1.73(1.64+0.09) -66.3% Note that there's another related case: we might have a filter that cannot be used with bitmaps. That check is handled already for us in for_each_bitmapped_object(), though we'd still load the bitmap and revindex files pointlessly in that case. I don't think it can happen in practice for cat-file, though, since it allows only blob:none, blob:limit, and object:type filters, all of which work with bitmaps. It would be easy-ish to insert an extra check like: can_filter_bitmap(&opt->objects_filter); into the conditional, but I didn't bother here. It would be redundant with the call in for_each_bitmapped_object(), and the can_filter helper function is static local in the bitmap code (so we'd have to make it public). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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