On Thu, Feb 28, 2019 at 08:36:58AM -0500, Michael S. Tsirkin wrote: > On Thu, Feb 14, 2019 at 03:39:12PM +1100, David Gibson wrote: > > When the balloon is inflated, we discard memory place in it using madvise() > > with MADV_DONTNEED. And when we deflate it we use MADV_WILLNEED, which > > sounds like it makes sense but is actually unnecessary. > > > > The misleadingly named MADV_DONTNEED just discards the memory in question, > > it doesn't set any persistent state on it in-kernel; all that's necessary > > to bring the memory back is to touch it. MADV_WILLNEED in contrast > > specifically says that the memory will be used soon and faults it in. > > > > This patch simplify's the balloon operation by dropping the madvise() > > on deflate. This might have an impact on performance - it will move a > > delay at deflate time until that memory is actually touched, which > > might be more latency sensitive. However: > > > > * Memory that's being given back to the guest by deflating the > > balloon *might* be used soon, but it equally could just sit around > > in the guest's pools until needed (or even be faulted out again if > > the host is under memory pressure). > > > > * Usually, the timescale over which you'll be adjusting the balloon > > is long enough that a few extra faults after deflation aren't > > going to make a difference. > > > > Signed-off-by: David Gibson > > Reviewed-by: David Hildenbrand > > Reviewed-by: Michael S. Tsirkin > > I'm having second thoughts about this. It might affect performance but > probably won't but we have no idea. Might cause latency jitter after > deflate where it previously didn't happen. This kind of patch should > really be accompanied by benchmarking results, not philosophy. I guess I see your point, much as it's annoying to spend time benchmarking a device that's basically broken by design. That said.. I don't really know how I'd go about benchmarking it. Any guesses at a suitable workload which would be most likely to show a performance degradation here? -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson