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Merge bitcoin/bitcoin#31645: [IBD] coins: increase default UTXO flush batch size to 32 MiB

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b6f8c48946cbfceb066de660c485ae1bd2c27cc1 coins: increase default `dbbatchsize` to 32 MiB (Lőrinc)
8bbb7b8bf8e3b2b6465f318ec102cc5275e5bf8c refactor: Extract default batch size into kernel (Lőrinc)

Pull request description:

  This change is part of [[IBD] - Tracking PR for speeding up Initial Block Download](https://github.com/bitcoin/bitcoin/pull/32043)

  ### Summary

  When the in-memory UTXO set is flushed to LevelDB (after IBD or AssumeUTXO load), it does so in batches to manage memory usage during the flush.
  A hidden `-dbbatchsize` config option exists to modify this value. This PR only changes the default from `16` MiB to `32` MiB.
  Using a larger default reduces the overhead of many small writes and improves I/O efficiency (especially on HDDs). It may also help LevelDB optimize writes more effectively (e.g., via internal ordering).
  The change is meant to speed up a critical part of IBD: dumping the accumulated work to disk.

  ### Context

  The UTXO set has grown significantly since [2017](https://github.com/bitcoin/bitcoin/pull/10148/files#diff-d102b6032635ce90158c1e6e614f03b50e4449aa46ce23370da5387a658342fdR26-R27), when the original fixed 16 MiB batch size was chosen.

  With the current multi-gigabyte UTXO set and the common practice of using larger `-dbcache` values, the fixed 16 MiB batch size leads to several inefficiencies:

  * Flushing the entire UTXO set often requires thousands of separate 16 MiB write operations.
  * Particularly on HDDs, the cumulative disk seek time and per-operation overhead from numerous small writes significantly slow down the flushing process.
  * Each `WriteBatch` call incurs internal LevelDB overhead (e.g., MemTable handling, compaction triggering logic). More frequent, smaller batches amplify this cumulative overhead.

  Flush times of 20-30 minutes are not uncommon, even on capable hardware.

  ### Considerations

  As [noted by sipa](https://github.com/bitcoin/bitcoin/pull/31645#issuecomment-2587500105), flushing involves a temporary memory usage increase as the batch is prepared. A larger batch size naturally leads to a larger peak during this phase. Crashing due to OOM during a flush is highly undesirable, but now that [#30611](https://github.com/bitcoin/bitcoin/pull/30611) is merged, the most we'd lose is the first hour of IBD.

  Increasing the LevelDB write batch size from 16 to 32 MiB raised the measured peaks by ~70 MiB in my tests during UTXO dump. The option remains hidden, and users can always override it.

  The increased peak memory usage (detailed below) is primarily attributed to LevelDB's `leveldb::Arena` (backing MemTables) and the temporary storage of serialized batch data (e.g., `std::string` in `CDBBatch::WriteImpl`).

  Performance gains are most pronounced on systems with slower I/O (HDDs), but some SSDs also show measurable improvements.

  ### Measurements:

  AssumeUTXO proxy, multiple runs with error bars (flushing time is faster that the measured loading + flushing):
  * Raspberry Pi, dbcache=500: ~30% faster with 32 MiB vs 16 MiB, peak +~75 MiB and still < 1 GiB.
  * i7 + HDD: results vary by dbcache, but 32 MiB usually beats 16 MiB and tracks close to 64 MiB without the larger peak.
  * i9 + fast NVMe: roughly flat across 16/32/64 MiB. The goal here is to avoid regressions, which holds.

  ### Reproducer:

  ```bash
  # Set up a clean demo environment
  rm -rfd demo && mkdir -p demo

  # Build Bitcoin Core
  cmake -B build -DCMAKE_BUILD_TYPE=Release && cmake --build build -j$(nproc)

  # Start bitcoind with minimal settings without mempool and internet connection
  build/bin/bitcoind -datadir=demo -stopatheight=1
  build/bin/bitcoind -datadir=demo -blocksonly=1 -connect=0 -dbcache=3000 -daemon

  # Load the AssumeUTXO snapshot, making sure the path is correct
  # Expected output includes `"coins_loaded": 184821030`
  build/bin/bitcoin-cli -datadir=demo -rpcclienttimeout=0 loadtxoutset ~/utxo-880000.dat

  # Stop the daemon and verify snapshot flushes in the logs
  build/bin/bitcoin-cli -datadir=demo stop
  grep "FlushSnapshotToDisk: completed" demo/debug.log
  ```

  ---

  This PR originally proposed 64 MiB, then a dynamic size, but both were dropped: 64 MiB increased peaks more than desired on low-RAM systems, and the dynamic variant underperformed across mixed hardware. 32 MiB is a simpler default that captures most of the gains with a modest peak increase.

  For more details see: https://github.com/bitcoin/bitcoin/pull/31645#issuecomment-3234329502
  ---

  While the PR isn't about IBD in general, rather about a critical section of it, I have measured a reindex-chainstate until 900k blocks, showing a 1% overall speedup:

  <details>
  <summary>Details</summary>

  ```python
  COMMITS="e6bfd95d5012fa1d91f83bf4122cb292afd6277f af653f321b135a59e38794b537737ed2f4a0040b"; \
  STOP=900000; DBCACHE=10000; \
  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' $c || exit 1; done; echo "") && \
  hyperfine \
    --sort command \
    --runs 1 \
    --export-json "$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 $DATA_DIR/debug.log; git checkout {COMMIT}; git clean -fxd; git reset --hard && \
      cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release && ninja -C build bitcoind && \
      ./build/bin/bitcoind -datadir=$DATA_DIR -stopatheight=$STOP -dbcache=1000 -printtoconsole=0; sleep 10" \
    --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"

  e6bfd95d50 Merge bitcoin-core/gui#881: Move `FreespaceChecker` class into its own module
  af653f321b coins: derive `batch_write_bytes` from `-dbcache` when unspecified

  Benchmark 1: COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=900000 -dbcache=10000 -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0 (COMMIT = e6bfd95d5012fa1d91f83bf4122cb292afd6277f)
    Time (abs ≡):        25016.346 s               [User: 30333.911 s, System: 826.463 s]

  Benchmark 2: COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=900000 -dbcache=10000 -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0 (COMMIT = af653f321b135a59e38794b537737ed2f4a0040b)
    Time (abs ≡):        24801.283 s               [User: 30328.665 s, System: 834.110 s]

  Relative speed comparison
          1.01          COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=900000 -dbcache=10000 -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0 (COMMIT = e6bfd95d5012fa1d91f83bf4122cb292afd6277f)
          1.00          COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=900000 -dbcache=10000 -reindex-chainstate -blocksonly -connect=0 -printtoconsole=0 (COMMIT = af653f321b135a59e38794b537737ed2f4a0040b)
  ```

  </details>

ACKs for top commit:
  laanwj:
    Concept and code review ACK b6f8c48946cbfceb066de660c485ae1bd2c27cc1
  TheCharlatan:
    ACK b6f8c48946cbfceb066de660c485ae1bd2c27cc1
  andrewtoth:
    ACK b6f8c48946cbfceb066de660c485ae1bd2c27cc1

Tree-SHA512: a72008feca866e658f0cb4ebabbeee740f9fb13680e517b9d95eaa136e627a9dd5ee328456a2bf040401f4a1977ffa7446ad13f66b286b3419ff0c35095a3521
This commit is contained in:
merge-script 2025-10-31 13:13:53 -04:00
commit da6f041e39
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3 changed files with 8 additions and 8 deletions
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2
src/init.cpp
View File

@ -496,7 +496,7 @@ void SetupServerArgs(ArgsManager& argsman, bool can_listen_ipc)
argsman.AddArg("-coinstatsindex", strprintf("Maintain coinstats index used by the gettxoutsetinfo RPC (default: %u)", DEFAULT_COINSTATSINDEX), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
argsman.AddArg("-conf=<file>", strprintf("Specify path to read-only configuration file. Relative paths will be prefixed by datadir location (only useable from command line, not configuration file) (default: %s)", BITCOIN_CONF_FILENAME), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
argsman.AddArg("-datadir=<dir>", "Specify data directory", ArgsManager::ALLOW_ANY | ArgsManager::DISALLOW_NEGATION, OptionsCategory::OPTIONS);
argsman.AddArg("-dbbatchsize", strprintf("Maximum database write batch size in bytes (default: %u)", nDefaultDbBatchSize), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::OPTIONS);
argsman.AddArg("-dbbatchsize", strprintf("Maximum database write batch size in bytes (default: %u)", DEFAULT_DB_CACHE_BATCH), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::OPTIONS);
argsman.AddArg("-dbcache=<n>", strprintf("Maximum database cache size <n> MiB (minimum %d, default: %d). Make sure you have enough RAM. In addition, unused memory allocated to the mempool is shared with this cache (see -maxmempool).", MIN_DB_CACHE >> 20, DEFAULT_DB_CACHE >> 20), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
argsman.AddArg("-includeconf=<file>", "Specify additional configuration file, relative to the -datadir path (only useable from configuration file, not command line)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
argsman.AddArg("-allowignoredconf", strprintf("For backwards compatibility, treat an unused %s file in the datadir as a warning, not an error.", BITCOIN_CONF_FILENAME), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);

3
src/kernel/caches.h
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@ -11,6 +11,9 @@
//! Suggested default amount of cache reserved for the kernel (bytes)
static constexpr size_t DEFAULT_KERNEL_CACHE{450_MiB};
//! Default LevelDB write batch size
static constexpr size_t DEFAULT_DB_CACHE_BATCH{32_MiB};
//! Max memory allocated to block tree DB specific cache (bytes)
static constexpr size_t MAX_BLOCK_DB_CACHE{2_MiB};
//! Max memory allocated to coin DB specific cache (bytes)

11
src/txdb.h
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@ -8,6 +8,7 @@
#include <coins.h>
#include <dbwrapper.h>
#include <kernel/caches.h>
#include <kernel/cs_main.h>
#include <sync.h>
#include <util/fs.h>
@ -21,16 +22,12 @@
class COutPoint;
class uint256;
//! -dbbatchsize default (bytes)
static const int64_t nDefaultDbBatchSize = 16 << 20;
//! User-controlled performance and debug options.
struct CoinsViewOptions {
//! Maximum database write batch size in bytes.
size_t batch_write_bytes = nDefaultDbBatchSize;
//! If non-zero, randomly exit when the database is flushed with (1/ratio)
//! probability.
int simulate_crash_ratio = 0;
size_t batch_write_bytes{DEFAULT_DB_CACHE_BATCH};
//! If non-zero, randomly exit when the database is flushed with (1/ratio) probability.
int simulate_crash_ratio{0};
};
/** CCoinsView backed by the coin database (chainstate/) */