From: Willy Tarreau <w@1wt.eu>
Date: Wed, 11 Mar 2026 15:33:33 +0000 (+0100)
Subject: MINOR: memprof: attempt different retry slots for different hashes on collision
X-Git-Tag: v3.4-dev7~70
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=fb7e5e169654e2c78d85774091ab92952b33f4f8;p=thirdparty%2Fhaproxy.git

MINOR: memprof: attempt different retry slots for different hashes on collision

When two pointer hash to the same memprofile bin, we currently try again
with the same bin until we find a spare one or we reach the limit of 16.
Olivier suggested to try with a different step for different pointers so
as to limit the number of bins to visit in such a case, so let's split
the pointer hash calculation so that we keep the raw hash before reduction
and use its lowest bits as the retry step. We force lowest bit to 1 to
avoid integral multiples that would oscillate between only a few positions.

Quick tests with h1+h2 requests show that for ~744 distinct entries, we
used to have 1.17 retries per lookup before and 0.6 now so we're halving
the cost of hash collisions. A heavier workload that used to produce 920
entries with 2.01 retries per lookup now reaches 966 entries (94.3% usage
vs 89.8% before) with only 1.44 retries per lookup.

This should be safe to backport, but depends on this previous commit:

    MINOR: tools: extend the pointer hashing code to ease manipulations
---

diff --git a/src/activity.c b/src/activity.c
index c55e3136f..d270a6500 100644
--- a/src/activity.c
+++ b/src/activity.c
@@ -302,13 +302,15 @@ struct memprof_stats *memprof_get_bin(const void *ra, enum memprof_method meth)
 	int retries = 16; // up to 16 consecutive entries may be tested.
 	const void *old;
 	unsigned int bin;
+	ullong hash;
 
 	if (unlikely(!ra)) {
 		bin = MEMPROF_HASH_BUCKETS;
 		goto leave;
 	}
-	bin = ptr_hash(ra, MEMPROF_HASH_BITS);
-	for (; memprof_stats[bin].caller != ra; bin = (bin + 1) & (MEMPROF_HASH_BUCKETS - 1)) {
+	hash = _ptr_hash(ra);
+	bin = _ptr_hash_reduce(hash, MEMPROF_HASH_BITS);
+	for (; memprof_stats[bin].caller != ra; bin = (bin + (hash | 1)) & (MEMPROF_HASH_BUCKETS - 1)) {
 		if (!--retries) {
 			bin = MEMPROF_HASH_BUCKETS;
 			break;