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gossmap: use mmap directly to check checksums.

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Instead of making a copy.

To measure the performance impact, I timed
tests/test_askrene.py::test_real_biases on my laptop.

	No checksum check: 194.52s
	Copying for checksum check: 202.81s
	Zero-copy checksum check: 194.40s

But these numbers proved noisy.  Still, doesn't hurt.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell 2025-02-10 12:18:49 +10:30 • committed by Alex Myers
parent 4b5e5b27ae
commit 05bc4ca5f3
1 changed files with 21 additions and 4 deletions
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25
common/gossmap.c
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@ -654,12 +654,26 @@ static bool reopen_store(struct gossmap *map, u64 ended_off)
return gossmap_refresh(map);
}
/* Extra sanity check (if it's cheap): does crc match? */
static bool csum_matches(const struct gossmap *map,
u64 off,
u32 timestamp,
u16 msglen,
u32 crc)
{
/* Don't waste resources reading if we don't have mmap */
if (!map->mmap)
return true;
assert(off + msglen <= map->map_size);
return crc32c(timestamp, map->mmap + off, msglen) == crc;
}
/* Returns false only if unknown_cb returns false */
static bool map_catchup(struct gossmap *map)
{
size_t reclen;
bool changed = false;
u8 msgbuf[65535];
for (; map->map_end + sizeof(struct gossip_hdr) < map->map_size;
map->map_end += reclen) {
@ -692,9 +706,12 @@ static bool map_catchup(struct gossmap *map)
off = map->map_end + sizeof(ghdr);
type = map_be16(map, off);
map_copy(map, off, msgbuf, msglen);
if (be32_to_cpu(ghdr.crc)
!= crc32c(be32_to_cpu(ghdr.timestamp), msgbuf, msglen)) {
if (!csum_matches(map, off,
be32_to_cpu(ghdr.timestamp),
msglen,
be32_to_cpu(ghdr.crc))) {
u8 msgbuf[65535];
map_copy(map, off, msgbuf, msglen);
map->logcb(map->cbarg,
LOG_BROKEN,
"Bad checksum on gossmap record @%"PRIu64