On 04.08.2010, at 17:48, Gleb Natapov wrote: > On Wed, Aug 04, 2010 at 05:31:12PM +0200, Alexander Graf wrote: >> >> On 04.08.2010, at 17:25, Gleb Natapov wrote: >> >>> On Wed, Aug 04, 2010 at 09:57:17AM -0500, Anthony Liguori wrote: >>>> On 08/04/2010 09:51 AM, David S. Ahern wrote: >>>>> >>>>> On 08/03/10 12:43, Avi Kivity wrote: >>>>>> libguestfs does not depend on an x86 architectural feature. >>>>>> qemu-system-x86_64 emulates a PC, and PCs don't have -kernel. We should >>>>>> discourage people from depending on this interface for production use. >>>>> That is a feature of qemu - and an important one to me as well. Why >>>>> should it be discouraged? You end up at the same place -- a running >>>>> kernel and in-ram filesystem; why require going through a bootloader >>>>> just because the hardware case needs it? >>>> >>>> It's smoke and mirrors. We're still providing a boot loader it's >>>> just a little tiny one that we've written soley for this purpose. >>>> >>>> And it works fine for production use. The question is whether we >>>> ought to be aggressively optimizing it for large initrd sizes. To >>>> be honest, after a lot of discussion of possibilities, I've come to >>>> the conclusion that it's just not worth it. >>>> >>>> There are better ways like using string I/O and optimizing the PIO >>>> path in the kernel. That should cut down the 1s slow down with a >>>> 100MB initrd by a bit. But honestly, shaving a couple hundred ms >>>> further off the initrd load is just not worth it using the current >>>> model. >>>> >>> The slow down is not 1s any more. String PIO emulation had many bugs >>> that were fixed in 2.6.35. I verified how much time it took to load 100M >>> via fw_cfg interface on older kernel and on 2.6.35. On older kernels on >>> my machine it took ~2-3 second on 2.6.35 it took 26s. Some optimizations >>> that was already committed make it 20s. I have some code prototype that >>> makes it 11s. I don't see how we can get below that, surely not back to >>> ~2-3sec. >> >> What exactly is the reason for the slowdown? It can't be only boundary and permission checks, right? >> >> > The big part of slowdown right now is that write into memory is done > for each byte. It means for each byte we call kvm_write_guest() and > kvm_mmu_pte_write(). The second call is needed in case memory, instruction > is trying to write to, is shadowed. Previously we didn't checked for > that at all. This can be mitigated by introducing write cache and do > combined writes into the memory and unshadow the page if there is more > then one write into it. This optimization saves ~10secs. Currently string Ok, so you tackled that bit already. > emulation enter guest from time to time to check if event injection is > needed and read from userspace is done in 1K chunks, not 4K like it was, > but when I made reads to be 4K and disabled guest reentry I haven't seen > any speed improvements worth talking about. So what are we wasting those 10 seconds on then? Does perf tell you anything useful? Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
