>>> On 20.09.12 at 15:49, Konrad Rzeszutek Wilk <konrad.w...@oracle.com> wrote: > On Thu, Sep 20, 2012 at 12:48:41PM +0100, Jan Beulich wrote: >> >>> On 20.09.12 at 13:30, Oliver Chick <oliver.ch...@citrix.com> wrote: >> > The memory overhead, and fallback mode points are related: >> > -Firstly, it turns out that the overhead is actually 2.75MB, not 11MB >> > per device. I made a mistake (pointed out by Jan) as the maximum number >> > of requests that can fit into a single-page ring is 64, not 256. >> > -Clearly, this still scales linearly. So the problem of memory footprint >> > will occur with more VMs, or block devices. >> > -Whilst 2.75MB per device is probably acceptable (?), if we start using >> > multipage rings, then we might not want to have >> > BLKIF_MAX_PERS_REQUESTS_PER_DEVICE==__RING_SIZE, as this will cause the >> > memory overhead to increase. This is why I have implemented the >> > 'fallback' mode. With a multipage ring, it seems reasonable to want the >> > first $x$ grefs seen by blkback to be treated as persistent, and any >> > later ones to be non-persistent. Does that seem sensible? >> >> From a resource usage pov, perhaps. But this will get the guest >> entirely unpredictable performance. Plus I don't think 11Mb of > > Wouldn't it fall back to the older performance?
Right, but the guest can't really predict this. That may have significant impact if the performance difference is big enough. Jan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
