libxc: maintain a small, per-handle, cache of hypercall buffer memory

Constantly m(un)locking memory can have significant overhead on
systems with large numbers of CPUs. This was previously fixed by
20841:fbe8f32fa257 but this was dropped during the transition to
hypercall buffers.

Introduce a small cache of single page hypercall buffer allocations
which can be resused to avoid this overhead.

Add some statistics tracking to the hypercall buffer allocations.

The cache size of 4 was chosen based on these statistics since they
indicated that 2 pages was sufficient to satisfy all concurrent single
page hypercall buffer allocations seen during "xl create", "xl
shutdown" and "xl destroy" of both a PV and HVM guest therefore 4
pages should cover the majority of important cases.

This fixes http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=1719.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reported-by: Zheng, Shaohui <shaohui.zheng@intel.com>
Tested-by: Haitao Shan <maillists.shan@gmail.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/tools/libxc/xc_private.h b/tools/libxc/xc_private.h
index b01ca2419d..3687561df0 100644
--- a/tools/libxc/xc_private.h
+++ b/tools/libxc/xc_private.h
@@ -75,6 +75,28 @@ struct xc_interface_core {
     FILE *dombuild_logger_file;
     const char *currently_progress_reporting;
 
+    /*
+     * A simple cache of unused, single page, hypercall buffers
+     *
+     * Protected by a global lock.
+     */
+#define HYPERCALL_BUFFER_CACHE_SIZE 4
+    int hypercall_buffer_cache_nr;
+    void *hypercall_buffer_cache[HYPERCALL_BUFFER_CACHE_SIZE];
+
+    /*
+     * Hypercall buffer statistics. All protected by the global
+     * hypercall_buffer_cache lock.
+     */
+    int hypercall_buffer_total_allocations;
+    int hypercall_buffer_total_releases;
+    int hypercall_buffer_current_allocations;
+    int hypercall_buffer_maximum_allocations;
+    int hypercall_buffer_cache_hits;
+    int hypercall_buffer_cache_misses;
+    int hypercall_buffer_cache_toobig;
+
+    /* Low lovel OS interface */
     xc_osdep_info_t  osdep;
     xc_osdep_ops    *ops; /* backend operations */
     xc_osdep_handle  ops_handle; /* opaque data for xc_osdep_ops */
@@ -158,6 +180,11 @@ void xc__hypercall_bounce_post(xc_interface *xch, xc_hypercall_buffer_t *bounce)
 #define xc_hypercall_bounce_post(_xch, _name) xc__hypercall_bounce_post(_xch, HYPERCALL_BUFFER(_name))
 
 /*
+ * Release hypercall buffer cache
+ */
+void xc__hypercall_buffer_cache_release(xc_interface *xch);
+
+/*
  * Hypercall interfaces.
  */