Fix performance regression in qemu_get_ram_ptr
When the commit f471a17e9d869df3c6573f7ec02c4725676d6f3a converted the
ram_blocks structure to QLIST, it also removed the conditional check before
switching the current block at the beginning of the list.
In the common use case where ram_blocks has a few blocks with only one
frequently accessed (the main RAM), this has a performance impact as it
performs the useless list operations on each call (which are on a really
hot path).
On my machine emulation (ARM on amd64), this patch reduces the
percentage of CPU time spent in qemu_get_ram_ptr from 6.3% to 2.1% in the
profiling of a full boot.
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/exec.c b/exec.c
index d611100..81f08b7 100644
--- a/exec.c
+++ b/exec.c
@@ -2957,8 +2957,11 @@
QLIST_FOREACH(block, &ram_list.blocks, next) {
if (addr - block->offset < block->length) {
- QLIST_REMOVE(block, next);
- QLIST_INSERT_HEAD(&ram_list.blocks, block, next);
+ /* Move this entry to to start of the list. */
+ if (block != QLIST_FIRST(&ram_list.blocks)) {
+ QLIST_REMOVE(block, next);
+ QLIST_INSERT_HEAD(&ram_list.blocks, block, next);
+ }
return block->host + (addr - block->offset);
}
}
