testsuite: add perftest.py to compare two rsync builds' transfer speed#998
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A standalone dev tool (run directly, not via runtests.py) for catching performance regressions between rsync releases. Given two rsync binaries it builds one deterministic test tree -- heavy-tailed file sizes, a directory spine, symlinks, hard links and a spread of permission modes, modelled on the gentestdata generator -- then runs the two binaries ALTERNATELY for N loops, timing each transfer, and reports the mean and standard deviation per binary. Each loop times a full copy into an emptied destination and an incremental no-op against an already-synced one (rsync's scan/file-list/stat overhead, where many regressions hide); --mode selects. The first run of each binary is dropped to reduce page-cache impact, the run order alternates to cancel drift, and a B-vs-A slowdown is flagged only when it exceeds the run-to-run noise.
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A standalone dev tool (run directly, not via runtests.py) for catching performance regressions between rsync releases. Given two rsync binaries it builds one deterministic test tree -- heavy-tailed file sizes, a directory spine, symlinks, hard links and a spread of permission modes, modelled on the gentestdata generator -- then runs the two binaries ALTERNATELY for N loops, timing each transfer, and reports the mean and standard deviation per binary.
Each loop times a full copy into an emptied destination and an incremental no-op against an already-synced one (rsync's scan/file-list/stat overhead, where many regressions hide); --mode selects. The first run of each binary is dropped to reduce page-cache impact, the run order alternates to cancel drift, and a B-vs-A slowdown is flagged only when it exceeds the run-to-run noise.