[Qemu-devel] [PATCH] exec: avoid unnecessary cacheline bounce on ram_list.mru_block

[Qemu-devel] [PATCH] exec: avoid unnecessary cacheline bounce on ram_list.mru_block

[Paolo Bonzini]

Whenever the MRU cache hits for the list of RAM blocks, qemu_get_ram_block
does an unnecessary write that causes a processor cache line to bounce
from one core to another.  This causes a performance hit.

Reported-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 exec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/exec.c b/exec.c
index 54cd70a..439b128 100644
--- a/exec.c
+++ b/exec.c
@@ -860,7 +860,7 @@ static RAMBlock *qemu_get_ram_block(ram_addr_t addr)
 
     block = atomic_rcu_read(&ram_list.mru_block);
     if (block && addr - block->offset < block->max_length) {
-        goto found;
+        return block;
     }
     QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
         if (addr - block->offset < block->max_length) {
-- 
2.5.0

Re: [Qemu-devel] [PATCH] exec: avoid unnecessary cacheline bounce on ram_list.mru_block

[Michael Tokarev]

22.10.2015 14:51, Paolo Bonzini write:
> Whenever the MRU cache hits for the list of RAM blocks, qemu_get_ram_block
> does an unnecessary write that causes a processor cache line to bounce
> from one core to another.  This causes a performance hit.

Applied to -trivial.  A good one! :)

/mjt