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Add -maxblocksintransit: tunable per-peer block-download window

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The per-peer in-flight block window (MAX_BLOCKS_IN_TRANSIT_PER_PEER) was a
hardcoded 16. On a single, high-latency peer during IBD the transfer is
bandwidth-delay-product bound (window / RTT), so with tiny sub-checkpoint
blocks the window, not bandwidth, is the ceiling — measured ~4x throughput
going 16 -> 64 on a 350ms-RTT peer. Make it a runtime flag (default 16,
clamped 1..4096), logged at startup. No behavior change at the default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
DanS
2026-06-29 21:22:32 -05:00
parent 2419ed7bf7
commit 78ea2aac5b
3 changed files with 17 additions and 2 deletions
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9
src/init.cpp
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@@ -1439,6 +1439,15 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
else if (nRandomXVerifyThreads > MAX_SCRIPTCHECK_THREADS)
nRandomXVerifyThreads = MAX_SCRIPTCHECK_THREADS;
// Per-peer block-download window (see MAX_BLOCKS_IN_TRANSIT_PER_PEER). Raising this lifts
// the bandwidth-delay-product ceiling on high-latency peers during IBD. Clamp to a sane range.
MAX_BLOCKS_IN_TRANSIT_PER_PEER = GetArg("-maxblocksintransit", DEFAULT_MAX_BLOCKS_IN_TRANSIT_PER_PEER);
if (MAX_BLOCKS_IN_TRANSIT_PER_PEER < 1)
MAX_BLOCKS_IN_TRANSIT_PER_PEER = 1;
else if (MAX_BLOCKS_IN_TRANSIT_PER_PEER > 4096)
MAX_BLOCKS_IN_TRANSIT_PER_PEER = 4096;
LogPrintf("Per-peer max blocks in transit: %d\n", MAX_BLOCKS_IN_TRANSIT_PER_PEER);
fServer = GetBoolArg("-server", false);
//fprintf(stderr,"%s tik6\n", __FUNCTION__);

1
src/main.cpp
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@@ -90,6 +90,7 @@ static int64_t nTimeBestReceived = 0;
CWaitableCriticalSection csBestBlock;
CConditionVariable cvBlockChange;
int nScriptCheckThreads = 0;
int MAX_BLOCKS_IN_TRANSIT_PER_PEER = DEFAULT_MAX_BLOCKS_IN_TRANSIT_PER_PEER;
int nRandomXVerifyThreads = 0; // parallel RandomX pre-verification worker count (0 = inline only)
bool fExperimentalMode = true;
bool fImporting = false;

9
src/main.h
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@@ -95,8 +95,13 @@ static const unsigned int UNDOFILE_CHUNK_SIZE = 0x100000; // 1 MiB
static const int MAX_SCRIPTCHECK_THREADS = 16;
/** -par default (number of script-checking threads, 0 = auto) */
static const int DEFAULT_SCRIPTCHECK_THREADS = 0;
/** Number of blocks that can be requested at any given time from a single peer. */
static const int MAX_BLOCKS_IN_TRANSIT_PER_PEER = 16;
/** Number of blocks that can be requested at any given time from a single peer.
* Runtime-tunable via -maxblocksintransit. The default of 16 caps single-peer IBD
* throughput at (window / RTT): on a high-latency peer with tiny (sub-checkpoint)
* blocks the transfer is bandwidth-delay-product bound, so a larger window lifts the
* ceiling at negligible bandwidth cost. */
static const int DEFAULT_MAX_BLOCKS_IN_TRANSIT_PER_PEER = 16;
extern int MAX_BLOCKS_IN_TRANSIT_PER_PEER;
/** Timeout in seconds during which a peer must stall block download progress before being disconnected. */
static const unsigned int BLOCK_STALLING_TIMEOUT = 2;
/** Number of headers sent in one getheaders result. We rely on the assumption that if a peer sends