From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=39632 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PxmmM-0000ki-KR for qemu-devel@nongnu.org; Thu, 10 Mar 2011 15:48:15 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PxmmL-0007DM-3j for qemu-devel@nongnu.org; Thu, 10 Mar 2011 15:48:14 -0500 Received: from smtp-out.google.com ([216.239.44.51]:12985) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PxmmK-0007CV-W9 for qemu-devel@nongnu.org; Thu, 10 Mar 2011 15:48:13 -0500 From: Vincent Palatin Date: Thu, 10 Mar 2011 15:47:46 -0500 Message-Id: <1299790066-768-1-git-send-email-vpalatin@chromium.org> Subject: [Qemu-devel] [PATCH] Fix performance regression in qemu_get_ram_ptr List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Qemu devel Cc: Chris Wright , Alex Williamson , Vincent Palatin , Anthony Liguori 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 --- exec.c | 7 +++++-- 1 files changed, 5 insertions(+), 2 deletions(-) diff --git a/exec.c b/exec.c index d611100..81f08b7 100644 --- a/exec.c +++ b/exec.c @@ -2957,8 +2957,11 @@ void *qemu_get_ram_ptr(ram_addr_t addr) 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); } } -- 1.7.3.1