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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/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