From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=54564 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Pxnmw-0001vK-RC for qemu-devel@nongnu.org; Thu, 10 Mar 2011 16:52:55 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Pxnmv-0001Za-He for qemu-devel@nongnu.org; Thu, 10 Mar 2011 16:52:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:25716) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Pxnmv-0001ZR-5y for qemu-devel@nongnu.org; Thu, 10 Mar 2011 16:52:53 -0500 Date: Thu, 10 Mar 2011 13:52:49 -0800 From: Chris Wright Message-ID: <20110310215249.GA16879@x200.localdomain> References: <1299790066-768-1-git-send-email-vpalatin@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1299790066-768-1-git-send-email-vpalatin@chromium.org> Subject: [Qemu-devel] Re: [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: Vincent Palatin Cc: Chris Wright , Alex Williamson , Qemu devel , Anthony Liguori * Vincent Palatin (vpalatin@chromium.org) wrote: > 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. Nice catch. > 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. Hopefully this is back on par with before the QLIST switchover. > Signed-off-by: Vincent Palatin Acked-by: Chris Wright > --- > 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); > + } Pretty close to self-documenting code now. Not sure if it's subtle enough to warrant change to the comment like: /* Move block to head of list if it's not there already */ thanks, -chris