Re: Dom0 starvs DomU

[David Brownlee <abs%absd.org@localhost>]

On 2 November 2015 at 21:39, Torbjörn Granlund <tg%gmplib.org@localhost> wrote:
> Greg Troxel <gdt%ir.bbn.com@localhost> writes:
>
>   tg%gmplib.org@localhost (Torbjörn Granlund) writes:
>
>   > I'm running NetBSD 7.0 with Xen 4.5, but in my experience this is true
>   > for any NetBSD version using any Xen version from pkgsrc
>   >
>   > When compiling in Dom0 (say, a new xentools version) all DomU's
>   > essentially freeze.  I think DomU's which do not touch disk keep
>   > running, but guest disk operations seem completely starved by CPU use in
>   > Dom0.
>   >
>   > I suppose this behaviour is well-known.  Are there plans of fixing it.
>   >
>   > It is worth noticing that Linux+Xen does not show the same problem.
>
>   This is not normal and I have not noticed experiencing it with
>   netbsd/xen.
>   It may just be that I haven't noticed it.
>
>   Could you post a recipe to reproduce so others can try?
>
> Load the CPU in Dom0, e.g., like this:
>
>   cd usr/pkgsrc/some/package
>   make [update|install] clean clean-depends
>
> While that runs, try
>
>   ls -l /usr/bin
>
> in some DomU (it doesn't matter if it is NetBSD or some other *nix, if
> its is HVM or PV.)
>
> Expect it to give < 1 io-operation/s.
>
> ("Don't crunch in Domo then".  Well, I don't.  But upgrading xen to plug
> the latest critical issues is quite important...)

I have a NetBSD/amd64 Xen Dom0 which hosts about ten Linux DomU
instances and it (occasionally) does full pkgsrc rebuilds in the Dom0.
If you're trying to do something very heavy in a DomU while (say)
libreoffice source is extracting it bogs down, but otherwise it seems
OK.

This has switched from a very old pair of SATA disks in a RAIDframe
RAID1 to a cheap Crucial SSD, and from netbsd-6 xen 4.1 through 4.2
and now 4.5 on netbsd-7.

The NetBSD filesystems are mounted -o log, in case that helps...