This change is part of [IBD] - Tracking PR for speeding up Initial Block Download
Summary
Previously, when the parent coins cache had no entry and the child did, BatchWrite performed a find followed by try_emplace, which resulted in multiple SipHash computations and bucket traversals on the common insert path.
On a different path, these caches were recreated needlessly for every block connection.
Fix for double fetch
This change uses a single leading try_emplace and branches on the returned inserted flag. In the FRESH && SPENT case (not used in production, only exercised by tests), we erase the just-inserted placeholder (which is constant time with no rehash anyway). Semantics are unchanged for all valid parent/child state combinations.
This change is a minimal version of bitcoin/bitcoin@723c49b (#32128) and draws simplification ideas bitcoin/bitcoin@ae76ec7 (#30673) and #30326.
Fix for temporary cache recreation
Related to parent cache propagation, the second commit makes it possible to avoid destructuring-recreating-destructuring of these short-live parent caches created for each new block.
A few temporary CCoinsViewCache's are destructed right after the Flush(), therefore it is not necessary to call ReallocateCache to recreate them right before they're killed anyway.
This change was based on a subset of #28945, the original authors and relevant commenters were added as coauthors to this version.
Reindex-chainstate indicates ~1% speedup.
<details> <summary>Details</summary>
COMMITS="647cdb4f7e8041affed887e2325ee03a91078bb1 0b0c3293ffd75afb27dadc0b28426b40132a8c6b"; \
STOP=909090; DBCACHE=4500; \
CC=gcc; CXX=g++; \
BASE_DIR="/mnt/my_storage"; DATA_DIR="$BASE_DIR/BitcoinData"; LOG_DIR="$BASE_DIR/logs"; \
(echo ""; for c in $COMMITS; do git fetch -q origin $c && git log -1 --pretty='%h %s' $c || exit 1; done; echo "") && \
hyperfine \
--sort command \
--runs 2 \
--export-json "$BASE_DIR/rdx-$(sed -E 's/(\w{8})\w+ ?/\1-/g;s/-$//'<<<"$COMMITS")-$STOP-$DBCACHE-$CC.json" \
--parameter-list COMMIT ${COMMITS// /,} \
--prepare "killall bitcoind 2>/dev/null; rm -f $DATA_DIR/debug.log; git checkout {COMMIT}; git clean -fxd; git reset --hard && \
cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DENABLE_IPC=OFF && ninja -C build bitcoind && \
./build/bin/bitcoind -datadir=$DATA_DIR -stopatheight=$STOP -dbcache=1000 -printtoconsole=0; sleep 20" \
--cleanup "cp $DATA_DIR/debug.log $LOG_DIR/debug-{COMMIT}-$(date +%s).log" \
"COMPILER=$CC ./build/bin/bitcoind -datadir=$DATA_DIR -stopatheight=$STOP -dbcache=$DBCACHE -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0"
647cdb4f7e Merge bitcoin/bitcoin#33311: net: Quiet down logging when router doesn't support natpmp/pcp
0b0c3293ff validation: don't reallocate cache for short-lived CCoinsViewCache
Benchmark 1: COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=909090 -dbcache=4500 -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0 (COMMIT = 647cdb4f7e8041affed887e2325ee03a91078bb1)
Time (mean ± σ): 16233.508 s ± 9.501 s [User: 19064.578 s, System: 951.672 s]
Range (min … max): 16226.790 s … 16240.226 s 2 runs
Benchmark 2: COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=909090 -dbcache=4500 -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0 (COMMIT = 0b0c3293ffd75afb27dadc0b28426b40132a8c6b)
Time (mean ± σ): 16039.626 s ± 17.284 s [User: 18870.130 s, System: 950.722 s]
Range (min … max): 16027.405 s … 16051.848 s 2 runs
Relative speed comparison
1.01 ± 0.00 COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=909090 -dbcache=4500 -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0 (COMMIT = 647cdb4f7e8041affed887e2325ee03a91078bb1)
1.00 COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=909090 -dbcache=4500 -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0 (COMMIT = 0b0c3293ffd75afb27dadc0b28426b40132a8c6b)
</details>
