common/buffer: switch crc cache to single pair instead of map

Most bufferptrs have just one entry in their caches, so it makes
perfect sense to store just that. As population and destruction of
std::map isn't free, it yields noticeable perf increase, around 12%
in case of bufferptrs containing just 4k of data, with more as the
bufferptrs get smaller.

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>

diff --git a/src/common/buffer.cc b/src/common/buffer.cc
index dc891e017460f07af197e97331390138b0a56ac4..48208bf38befc73e2d57a51a091b194aa03bf6bd 100644 (file)
--- a/src/common/buffer.cc
+++ b/src/common/buffer.cc
@@ -173,8 +173,10 @@ using namespace ceph;
     std::atomic<unsigned> nref { 0 };
     int mempool;
 
+    std::pair<size_t, size_t> last_crc_offset {std::numeric_limits<size_t>::max(), std::numeric_limits<size_t>::max()};
+    std::pair<uint32_t, uint32_t> last_crc_val;
+
     mutable ceph::spinlock crc_spinlock;
-    map<pair<size_t, size_t>, pair<uint32_t, uint32_t> > crc_map;
 
     explicit raw(unsigned l, int mempool=mempool::mempool_buffer_anon)
       : data(NULL), len(l), nref(0), mempool(mempool) {
@@ -248,24 +250,22 @@ public:
     bool get_crc(const pair<size_t, size_t> &fromto,
          pair<uint32_t, uint32_t> *crc) const {
       std::lock_guard<decltype(crc_spinlock)> lg(crc_spinlock);
-      map<pair<size_t, size_t>, pair<uint32_t, uint32_t> >::const_iterator i =
-      crc_map.find(fromto);
-      if (i == crc_map.end()) {
-          return false;
+      if (last_crc_offset == fromto) {
+        *crc = last_crc_val;
+        return true;
       }
-      *crc = i->second;
-      return true;
+      return false;
     }
     void set_crc(const pair<size_t, size_t> &fromto,
          const pair<uint32_t, uint32_t> &crc) {
       std::lock_guard<decltype(crc_spinlock)> lg(crc_spinlock);
-      crc_map[fromto] = crc;
+      last_crc_offset = fromto;
+      last_crc_val = crc;
     }
     void invalidate_crc() {
       std::lock_guard<decltype(crc_spinlock)> lg(crc_spinlock);
-      if (crc_map.size() != 0) {
-        crc_map.clear();
-      }
+      last_crc_offset.first = std::numeric_limits<size_t>::max();
+      last_crc_offset.second = std::numeric_limits<size_t>::max();
     }
   };