Re: [Libguestfs] [PATCH v2v] v2v: Set the number of vCPUs to same as host number of pCPUs.

On Fri, Sep 18, 2020 at 10:52:58AM +0100, Daniel P. Berrangé wrote: > On Fri, Sep 18, 2020 at 10:44:03AM +0100, Richard W.M. Jones wrote: > > So it didn't make any noticable difference in my test. I wonder if > > the test guest I'm using (Fedora 32 using dracut) doesn't use parallel > > compression? > > Do you do anything special to optimize storage ? If the thing using > parallel CPUs in the guest is doing I/O you'd likely want to tune > storage at same time. In current QEMU git master there's a change[1] > that sets viort-blk/virtio-scsi num_queues to match SMP count, > so the guest OS get fully parallel I/O from meach guest CPU. > > [1] a4eef0711b2cf7a7476c3e2c202a414b68a1baa0 I'm not using a new enough qemu to have that and it doesn't appear in a released version, but I'll test again once we have a new enough version.

> > However I don't think it can cause a problem and it seems obvious that > > it could benefit some cases. > > What's the default RAM size ? On hosts with very large numbers of CPUs > (100's) you might need to scale up RAM size too so you keep a sane > RAM-per-CPU ratio. Or default to host CPUs, but cap it at say 32. I'll cap the number of CPUs in this patch. Thanks, Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW