From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself Date: Thu, 20 May 2010 09:04:37 +0300 Message-ID: <4BF4D0F5.7050003@redhat.com> References: <20100505205814.GA7090@redhat.com> <201005071253.53393.rusty@rustcorp.com.au> <4BE9AF9A.8080005@redhat.com> <201005191709.16401.rusty@rustcorp.com.au> <4BF39C12.7090407@redhat.com> <20100519223312.GC4111@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Rusty Russell , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, qemu-devel@nongnu.org To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20100519223312.GC4111@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 05/20/2010 01:33 AM, Michael S. Tsirkin wrote: > >> Virtio is already way too bouncy due to the indirection between the >> avail/used rings and the descriptor pool. A device with out of order >> completion (like virtio-blk) will quickly randomize the unused >> descriptor indexes, so every descriptor fetch will require a bounce. >> >> In contrast, if the rings hold the descriptors themselves instead of >> pointers, we bounce (sizeof(descriptor)/cache_line_size) cache lines for >> every descriptor, amortized. >> > On the other hand, consider that on fast path we are never using all > of the ring. With a good allocator we might be able to keep > reusing only small part of the ring, instead of wrapping around > all of it all of the time. > It's still suboptimal, we have to bounce both the avail/used rings and the descriptor pool, compared to just the descriptor ring with a direct design. Plus we don't need a fancy allocator. When amortizing cachelines, simple data structures win. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.