On 19.07.2010, at 10:01, Gleb Natapov wrote: > On Mon, Jul 19, 2010 at 09:57:02AM +0200, Alexander Graf wrote: >> >> On 19.07.2010, at 09:51, Gleb Natapov wrote: >> >>> On Mon, Jul 19, 2010 at 09:40:18AM +0200, Alexander Graf wrote: >>>> >>>> On 19.07.2010, at 09:33, Gleb Natapov wrote: >>>> >>>>> On Mon, Jul 19, 2010 at 08:28:02AM +0100, Richard W.M. Jones wrote: >>>>>> On Mon, Jul 19, 2010 at 09:23:56AM +0300, Gleb Natapov wrote: >>>>>>> That what I am warring about too. If we are adding device we have to be >>>>>>> sure such device can actually exist on real hw too otherwise we may have >>>>>>> problems later. >>>>>> >>>>>> I don't understand why the constraints of real h/w have anything to do >>>>>> with this. Can you explain? >>>>>> >>>>> Each time we do something not architectural it cause us troubles later. >>>>> So constraints of real h/w is our constrains to. >>>>> >>>>>>> Also 1 second on 100M file does not look like huge gain to me. >>>>>> >>>>>> Every second counts. We're trying to get libguestfs boot times down >>>>>> from 8-12 seconds to 4-5 seconds. For many cases it's an interactive >>>>>> program. >>>>>> >>>>> So what about making initrd smaller? I remember managing two >>>>> distribution in 64M flash in embedded project. >>>> >>>> Having a huge initrd basically helps in reusing a lot of existing code. We >>>> do the same - in general the initrd is just a subset of the applications >>>> of the host OS. And if you start putting perl or the likes into it, it >>>> becomes big. >>>> >>> Why not provide small disk/cdrom with all those utilities installed? >> >> Because - if the loading is done fast - this way everything's in RAM >> instantly. And you still have all devices available for use inside the >> system - that makes enumeration a lot easier. There are several reasons why >> and I don't think we should force different ways on people just because one >> component of our system is ineffective. >> > Loading huge initrd on real HW takes noticeably longer time that small > one, so I would say that it is your design that is to blame here, not > KVM.
I disagree. Virtualization enables new use cases. The -initrd parameter is a very good example for that. It's something that you simply couldn't do on real hw. > >>> >>>> I guess the best thing for now really is to try and see which code paths >>>> insb goes along. It should really be coalesced. >>>> >>> It is coalesced to a certain extent (reenter guest every 1024 bytes, >>> read from userspace page at a time). You need to continue injecting >>> interrupt into a guest during long string operation and checking >>> exception condition on a page boundaries. >> >> That still sounds slow. So yeah, adding DMA is probably the right way to go. >> But then again - if we model it after real hw it would be asynchronous, >> giving us an interrupt, causing even more headache. Ugh. >> >> Can't we just ignore real hw constraints here and have it available in guest >> ram once one particular PIO is done? No bus master, no interrupts, but full >> speed and simplicity/atomicity which also helps migration. >> > We shouldn't add devices that work not like real HW to speed up some > pathological cases (and are slow on real HW too). Just because you don't use them doesn't mean they're pathological, really. We simply chose a bad interface for transferring reasonable big chunks of data and we need to fix that. If you want to look at it from a different perspective, it's a regression. Older qemu versions did map the kernel and initrd directly into guest ram, so now we're slower than back then. Alex
