On Tue, Mar 05, 2019 at 03:15:38PM +0100, David Hildenbrand wrote: > On 05.03.19 06:11, David Gibson wrote: > > Prior to f6deb6d9 "virtio-balloon: Remove unnecessary MADV_WILLNEED on > > deflate", the balloon device issued an madvise() MADV_WILLNEED on > > pages removed from the balloon. That would hint to the host kernel > > that the pages were likely to be needed by the guest in the near > > future. > > > > It's unclear if this is actually valuable or not, and so f6deb6d9 > > removed this, essentially ignoring balloon deflate requests. However, > > concerns have been raised that this might cause a performance > > regression by causing extra latency for the guest in certain > > configurations. > > I mean, it will mainly create page tables as far as I know. Any write to > a page will have an overhead either way (COW zero page). Reads *might* > be faster. > > As we are working on 4k granularity in the balloon (and doing > MADV_DONTNEED on 4k granularity!), there will most probably be page > tables already either way. A page table could only be zapped if all > pages of that page table are MADV_DONTNEED'ed (or I assume never were > touched), and I am not sure if "random MADV_DONTNEED'ing of 4k pages" > will actually get rid of page tables (my assumption would be: only if a > complete range is zapped at once). I haven't looked into the details, > though (plenty of other stuff to do). > > I am not sure if I share the concerns. Real-time workload should never > use the virtio-balloon in a way that anything like that would be possible. > > > > > So, until we can get actual benchmark data to see if that's the case, > > this restores (by default) the old behaviour, issuing a MADV_WILLNEED > > when a page is removed from the balloon. A new property on the > > balloon device "hint-on-deflate" can be set to false to remove this > > behaviour for testing. > > This is certainly a good approach for you to finally be able to leave > the ugly land of virtio-balloon :) > > But at least to me, this looks completely useless. I'll be happy to be > proven wrong as always :)
Frankly, I agree. But mst was nervous about us making that change to
the balloon's previous behaviour.
--
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
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