scheduleFetches.func1 is the single biggest allocator in the Pyroscope
profile of a busy node (~13.5 GB/hr, 8% of total alloc_space). Each
peer-iteration pre-allocated 'make([]common.Hash, 0, maxTxRetrievals)'
= 8 KB, even for peers that end up collecting no new hashes (all their
announces were already being fetched by someone else).
Defer the slice allocation to the first append. Peers that collect zero
hashes now pay zero allocation, which is the common case on the
timeoutTrigger path where all peers with any announces are iterated.
New benchmarks BenchmarkScheduleFetches_{100peers_10new,
100peers_allFetching, 500peers_3new} (benchstat, 6 samples):
scenario ns/op B/op allocs/op
100p/10new unchanged unchanged unchanged (fast path)
100p/allFetching -62% -92% -20%
500p/3new -22% -44% -7%
geomean -33% -65% -9%
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