Re: Some performance numbers for virtiofs, DAX and virtio-9p

[Vivek Goyal]

On Fri, Dec 11, 2020 at 06:29:56PM +0000, Dr. David Alan Gilbert wrote:

[..]
> > > 
> > > Could we measure at what point does a large window size actually make
> > > performance worse?
> > 
> > Will do. Will run tests with varying window sizes (small to large)
> > and see how does it impact performance for same workload with
> > same guest memory.
> 
> I wonder how realistic it is though;  it makes some sense if you have a
> scenario like a fairly small root filesystem - something tractable;  but
> if you have a large FS you're not realistically going to be able to set
> the cache size to match it - that's why it's a cache!

I think its more about active dataset size and not necessarily total
FS size. FS might be big but if application is not accessing all of
the in reasonabl small time window, then it does not matter.

What worries me most is that cost of reclaim of a dax range seems
too high (or keeps the process blocked for long enogh), that it
kills the performance. I will need to revisit the reclaim path
and see if I can optimize something.

Vivek