From: Remi Gacogne <remi.gacogne@powerdns.com>
Date: Fri, 23 Jan 2026 10:07:23 +0000 (+0100)
Subject: dnsdist: Reduce contention around the sampling counter in the rings
X-Git-Tag: dnsdist-2.1.0-alpha1~6^2~6
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=848ec456a2e1a4744267a90200ecf4cca60ee2a8;p=thirdparty%2Fpdns.git

dnsdist: Reduce contention around the sampling counter in the rings

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>
---

diff --git a/pdns/dnsdistdist/dnsdist-rings.cc b/pdns/dnsdistdist/dnsdist-rings.cc
index e1d9b607e1..8ac89cb786 100644
--- a/pdns/dnsdistdist/dnsdist-rings.cc
+++ b/pdns/dnsdistdist/dnsdist-rings.cc
@@ -24,6 +24,8 @@
 
 #include "dnsdist-rings.hh"
 
+thread_local size_t Rings::t_samplingCounter{0};
+
 void Rings::init(const RingsConfiguration& config)
 {
   if (d_initialized.exchange(true)) {
@@ -207,8 +209,8 @@ bool Rings::shouldSkipDueToSampling()
   if (d_samplingRate == 0) {
     return false;
   }
-  auto counter = d_samplingCounter++;
-  return (counter % d_samplingRate) == 0;
+  auto counter = t_samplingCounter++;
+  return (counter % d_samplingRate) != 0;
 }
 
 uint32_t Rings::adjustForSamplingRate(uint32_t count) const
diff --git a/pdns/dnsdistdist/dnsdist-rings.hh b/pdns/dnsdistdist/dnsdist-rings.hh
index 03378941fd..ef99dd2c7d 100644
--- a/pdns/dnsdistdist/dnsdist-rings.hh
+++ b/pdns/dnsdistdist/dnsdist-rings.hh
@@ -290,6 +290,9 @@ private:
   bool shouldSkipDueToSampling();
 
   static constexpr bool s_keepLockingStats{false};
+  // small hack to reduce contention: this only works because we have a single Rings object in DNSdist
+  static thread_local size_t t_samplingCounter;
+
 
   std::atomic<size_t> d_nbQueryEntries{0};
   std::atomic<size_t> d_nbResponseEntries{0};
@@ -300,7 +303,6 @@ private:
   size_t d_numberOfShards{10};
   size_t d_nbLockTries{5};
   size_t d_samplingRate{0};
-  std::atomic<size_t> d_samplingCounter{0};
   bool d_recordQueries{true};
   bool d_recordResponses{true};
 };