Regression is due to decompression overhead. From build 7.1.0-1887 onwards, plasma will compress on swapin. If scans accesses only compressed pages, they will be slower due to decompression overhead. This can occur even when there is sufficient memory between usage and quota. To overcome this, compression on swapin can be performed only when there is memory pressure and with this patch, the performance for this test is back to normal. But, this can be bad for cases where the hot set does not change or gets swapped-out/swapped-in and the hot set does not fit in quota as in the case of the new compression tests. In these tests, regression is observed due to lower resident ratio from not compressing.

Build couchbase-server-7.1.0-2238 contains indexing commit 5415ba9 with commit message:
MB-50250: Add plasma CompressMemoryThreshold config

Build couchbase-server-7.1.0-2238 contains plasma commit 98fa535 with commit message:
MB-50250: Compress after swapin only if under memory pressure

This regression was seen in 2 tests:
1) 1000idx scan throughput test with collections
2) 1idx scan throughput test with bucket

With fix in build couchbase-server-7.1.0-2238, I can see that the number for (1) is back to normal. But (2) still regresses. It still shows regression due to decompression overhead. Will need something like MB-50771 to help with it. The reason for the threshold changes not helping is that after init build pages are compressed and they are never swapped-out/swapped-in during scan phase. This is not the case with the 1000idx test where the working set seems to get swapped-in during scan phase and the threshold fix avoids compression here.

Back to normal with 7.1.0-2238