(impala) 02/02: IMPALA-15585: Detect fast CLOCK_MONOTONIC on ARM

[jo...@apache.org]

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commit 64a7b8b8b9ab0676e37ade0663473fc6e854b2d4
Author: Joe McDonnell <jo...@cloudera.com>
AuthorDate: Sun Oct 18 22:09:06 2026 -0700

    IMPALA-15585: Detect fast CLOCK_MONOTONIC on ARM
    
    In IMPALA-2407, Impala added logic to detect whether clock_gettime's
    CLOCK_MONOTONIC has a fast implementation. At the time, some clock
    implementations like Xen were very slow. To avoid the performance
    penalty, it uses CLOCK_MONOTONIC_COARSE when the clock source
    is not known to be fast.
    
    ARM has a fast clock source for CLOCK_MONOTONIC, but it is not being
    detected properly because it has a different name than x86_64.
    ARM currently uses CLOCK_MONOTONIC_COARSE, which makes profile
    time counters considerably less useful.
    
    This adds detection for the ARM fast time counter (arch_sys_counter)
    in addition to the x86_64 version (tsc).
    
    Testing:
     - Ran a perf-AB-test on ARM and verified that the profiles have
       more exact time values (without performance regression)
    
    Change-Id: I6512e5dbbb11c4816716deccce3c0e212f73cf33
    Reviewed-on: http://gerrit.cloudera.org:8080/25149
    Tested-by: Impala Public Jenkins <im...@cloudera.com>
    Reviewed-by: Yida Wu <wy...@gmail.com>
    Reviewed-by: Jason Fehr <jf...@cloudera.com>
---
 be/src/util/os-info.cc | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/be/src/util/os-info.cc b/be/src/util/os-info.cc
index 0811248c9..f7aaa52ae 100644
--- a/be/src/util/os-info.cc
+++ b/be/src/util/os-info.cc
@@ -21,6 +21,7 @@
 #include <string.h>
 #include <fstream>
 #include <sstream>
+#include <string_view>
 
 #include <unistd.h>
 #include <sys/stat.h>
@@ -87,17 +88,23 @@ void OsInfo::Init() {
   if (os_version.good()) getline(os_version, os_version_);
   if (os_version.is_open()) os_version.close();
 
-  // Read the current clocksource to see if CLOCK_MONOTONIC is known to be fast. "tsc" is
-  // fast, while "xen" is slow (40 times slower than "tsc" on EC2). If CLOCK_MONOTONIC is
+  // Read the current clocksource to see if CLOCK_MONOTONIC is known to be fast.
+  // On X86_64, "tsc" is fast, while "xen" is slow (40 times slower than "tsc"
+  // on EC2). On ARM, "arch_sys_counter" is the fast clocksource. If CLOCK_MONOTONIC is
   // known to be slow, we use CLOCK_MONOTONIC_COARSE, which uses jiffies, with a
   // resolution measured in milliseconds, rather than nanoseconds.
+#ifdef __aarch64__
+  constexpr std::string_view FAST_CLOCK_SOURCE = "arch_sys_counter";
+#else
+  constexpr std::string_view FAST_CLOCK_SOURCE = "tsc";
+#endif
   std::ifstream clocksource_file(
       "/sys/devices/system/clocksource/clocksource0/current_clocksource");
   if (clocksource_file.good()) {
     std::string clocksource;
     clocksource_file >> clocksource;
     clock_name_ = "clocksource: '" + clocksource + "', clockid_t: ";
-    if (HAVE_CLOCK_MONOTONIC_COARSE && clocksource != "tsc") {
+    if (HAVE_CLOCK_MONOTONIC_COARSE && clocksource != FAST_CLOCK_SOURCE) {
       clock_name_ += "CLOCK_MONOTONIC_COARSE";
       fast_clock_ = CLOCK_MONOTONIC_COARSE;
     } else {