[PATCH] memory: batch allocate ioeventfds[] in address_space_update_ioeventfds()

[Stefan Hajnoczi <stefanha@redhat.com>]

Reallocing the ioeventfds[] array each time an element is added is very
expensive as the number of ioeventfds increases.  Batch allocate instead
to amortize the cost of realloc.

This patch reduces Linux guest boot times from 362s to 140s when there
are 2 virtio-blk devices with 1 virtqueue and 99 virtio-blk devices with
32 virtqueues.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 memory.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/memory.c b/memory.c
index aeaa8dcc9e..2d6f931f8c 100644
--- a/memory.c
+++ b/memory.c
@@ -794,10 +794,18 @@ static void address_space_update_ioeventfds(AddressSpace *as)
     FlatView *view;
     FlatRange *fr;
     unsigned ioeventfd_nb = 0;
-    MemoryRegionIoeventfd *ioeventfds = NULL;
+    unsigned ioeventfd_max;
+    MemoryRegionIoeventfd *ioeventfds;
     AddrRange tmp;
     unsigned i;
 
+    /*
+     * It is likely that the number of ioeventfds hasn't changed much, so use
+     * the previous size as the starting value.
+     */
+    ioeventfd_max = as->ioeventfd_nb;
+    ioeventfds = g_new(MemoryRegionIoeventfd, ioeventfd_max);
+
     view = address_space_get_flatview(as);
     FOR_EACH_FLAT_RANGE(fr, view) {
         for (i = 0; i < fr->mr->ioeventfd_nb; ++i) {
@@ -806,8 +814,11 @@ static void address_space_update_ioeventfds(AddressSpace *as)
                                              int128_make64(fr->offset_in_region)));
             if (addrrange_intersects(fr->addr, tmp)) {
                 ++ioeventfd_nb;
-                ioeventfds = g_realloc(ioeventfds,
-                                          ioeventfd_nb * sizeof(*ioeventfds));
+                if (ioeventfd_nb > ioeventfd_max) {
+                    ioeventfd_max += 64;
+                    ioeventfds = g_realloc(ioeventfds,
+                            ioeventfd_max * sizeof(*ioeventfds));
+                }
                 ioeventfds[ioeventfd_nb-1] = fr->mr->ioeventfds[i];
                 ioeventfds[ioeventfd_nb-1].addr = tmp;
             }
-- 
2.24.1