[kudu] 01/03: tserver: avoid spinlock in scanners hot path

[al...@apache.org]

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alexey pushed a commit to branch master
in repository https://gitbox.apache.org/repos/asf/kudu.git

commit 9d2ae8b2114f0add8e63c6ccd63ac0b9a49079d6
Author: Todd Lipcon <to...@apache.org>
AuthorDate: Mon Mar 2 23:12:37 2020 -0800

    tserver: avoid spinlock in scanners hot path
    
    This replaces a spinlock protection of the scanner "rows_returned"
    counter to just be an atomic integer instead. This saved some CPU cycles
    in the TSBS benchmark.
    
    Change-Id: If3c32e94ada847076580e20571440833a8544c57
    Reviewed-on: http://gerrit.cloudera.org:8080/15502
    Reviewed-by: Adar Dembo <ad...@cloudera.com>
    Tested-by: Kudu Jenkins
---
 src/kudu/tserver/scanners.h | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/src/kudu/tserver/scanners.h b/src/kudu/tserver/scanners.h
index b30bcb9..5f81763 100644
--- a/src/kudu/tserver/scanners.h
+++ b/src/kudu/tserver/scanners.h
@@ -16,6 +16,7 @@
 // under the License.
 #pragma once
 
+#include <atomic>
 #include <cstddef>
 #include <cstdint>
 #include <memory>
@@ -299,13 +300,11 @@ class Scanner {
   }
 
   void add_num_rows_returned(int64_t num_rows_added) {
-    std::lock_guard<simple_spinlock> l(lock_);
     num_rows_returned_ += num_rows_added;
     DCHECK_LE(num_rows_added, num_rows_returned_);
   }
 
   int64_t num_rows_returned() const {
-    std::lock_guard<simple_spinlock> l(lock_);
     return num_rows_returned_;
   }
 
@@ -347,8 +346,7 @@ class Scanner {
   // The current call sequence ID.
   uint32_t call_seq_id_;
 
-  // Protects last_access_time_ call_seq_id_, iter_, spec_, and
-  // num_rows_returned_.
+  // Protects last_access_time_ call_seq_id_, iter_, spec_
   mutable simple_spinlock lock_;
 
   // The time the scanner was started.
@@ -383,7 +381,7 @@ class Scanner {
 
   // The number of rows that have been serialized and sent over the wire by
   // this scanner.
-  int64_t num_rows_returned_;
+  std::atomic<int64_t> num_rows_returned_;
 
   // The cumulative amounts of wall, user cpu, and system cpu time spent on
   // this scanner, in seconds.