[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] Re: [Qemu-devel] [PATCH 5/7] xen: add block device backend driver.
Blue Swirl wrote: On 8/4/08, Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:+/* i386 protocol version */ +#pragma pack(push, 4)What's wrong with __attribute__(__aligned__)?+ * FIXME: the code is designed to handle multiple outstanding + * requests (using aio or using threads), which isn't used right + * now due to limitations of the qemu block driver interface.Yes, what happened to vectored DMA patches? Believe it or not, they were hurting performance in KVM. Passing a vector to the block layer will force the IO operation to be split up into multiple requests that are all executed in serial (since QEMU only supports 1 outstanding request at a time). Copying the vectored IO to a linear buffer increased performance significantly because it avoided this serialization. The block layer needs some refactoring to support proper vectored AIO operations before a zero-copy API will make sense from a performance perspective. Regards, Anthony Liguori _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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