Re: [Qemu-devel] [PATCH 2/2 v2] Direct IDE I/O

[Samuel Thibault]

Paul Brook, le Mon 03 Dec 2007 15:39:48 +0000, a écrit :
> I think host caching is still useful enough to be enabled by default, and 
> provides a significant performance increase in several cases. 
> 
> - The guest typically has a relatively small quantity of RAM, compared to a 
> modern machine.  Allowing the host OS to act as a demand-based L2 cache 
> allows this to be used without having to dedicate excessive quantities of ram 
> to qemu.
> - I've seen reports that it significantly speeds up the windows installer.
> - Host cache is persistent between multiple qemu runs. f you're doing 
> anything 
> that requires frequent guest reboots (e.g. kernel debugging) this is going to 
> be a huge win.
> - You're running a host OS that has limited or no caching (e.g. DOS).

Yes, and in other cases (e.g. real-production KVM/Xen servers), this is
just cache duplication.

> I'd hope that the host OS would have cache use heuristics that would help 
> limit cache pollution.

How could it?  It can't detect that the guest also has a buffer/page
cache.

Samuel