>>> 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 _virtual_ space is unacceptable overhead in a 64-bit kernel. If you really want/need this in a 32-bit one, then perhaps some other alternatives would be needed (and persistent grants may not be the right approach there in the first place). 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/
