[Qemu-devel] [PATCH v3 4/4] coroutine: pool coroutines to speed up creation

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

This patch speeds up coroutine creation by reusing freed coroutines.
When a coroutine terminates it is placed in the pool instead of having
its resources freed.  The next time a coroutine is created it can be
taken straight from the pool and requires no initialization.

Performance results on an Intel Core2 Duo T9400 (2.53GHz) for
./check-coroutine --benchmark-lifecycle 20000000:

  No pooling:    19.5 sec
  With pooling:   1.1 sec

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 qemu-coroutine-int.h |    2 ++
 qemu-coroutine.c     |   33 ++++++++++++++++++++++++++++++---
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/qemu-coroutine-int.h b/qemu-coroutine-int.h
index 71c6ee9..b264881 100644
--- a/qemu-coroutine-int.h
+++ b/qemu-coroutine-int.h
@@ -45,6 +45,8 @@ struct Coroutine {
     void *data;
     CoroutineEntry *entry;
 
+    QLIST_ENTRY(Coroutine) pool_next;
+
     jmp_buf env;
 };
 
diff --git a/qemu-coroutine.c b/qemu-coroutine.c
index a80468b..0f4e58f 100644
--- a/qemu-coroutine.c
+++ b/qemu-coroutine.c
@@ -24,16 +24,35 @@
 #include "qemu-coroutine.h"
 #include "qemu-coroutine-int.h"
 
+enum {
+    /* Maximum free pool size prevents holding too many freed coroutines */
+    POOL_MAX_SIZE = 64,
+};
+
+static QLIST_HEAD(, Coroutine) pool = QLIST_HEAD_INITIALIZER(&pool);
+static unsigned int pool_size;
 static Coroutine leader;
 static Coroutine *current;
 
-static void coroutine_terminate(Coroutine *co)
+static void coroutine_delete(Coroutine *co)
 {
-    trace_qemu_coroutine_terminate(co);
     qemu_free(co->stack);
     qemu_free(co);
 }
 
+static void coroutine_terminate(Coroutine *co)
+{
+    trace_qemu_coroutine_terminate(co);
+
+    if (pool_size < POOL_MAX_SIZE) {
+        QLIST_INSERT_HEAD(&pool, co, pool_next);
+        co->caller = NULL;
+        pool_size++;
+    } else {
+        coroutine_delete(co);
+    }
+}
+
 static Coroutine *coroutine_new(void)
 {
     const size_t stack_size = 4 << 20;
@@ -49,7 +68,15 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
 {
     Coroutine *co;
 
-    co = coroutine_new();
+    co = QLIST_FIRST(&pool);
+
+    if (co) {
+        QLIST_REMOVE(co, pool_next);
+        pool_size--;
+    } else {
+        co = coroutine_new();
+    }
+
     co->entry = entry;
 
     return co;
-- 
1.7.4.4