Re: [PATCH] kvm: x86: increase user memory slots to 509

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/11/2014 15:10, Igor Mammedov wrote:
> On Thu, 06 Nov 2014 17:23:58 +0100 Paolo Bonzini <pbonzini@redhat.com> wrote:
>> It would use more memory, and some loops are now becoming more
>> expensive.  In general adding a memory slot to a VM is not cheap, and
>> I question the wisdom of having 256 hotplug memory slots.  But the
>> slowdown mostly would only happen if you actually _use_ those memory
>> slots, so it is not a blocker for this patch.
> It might be useful to have a big amount of slots for big guests
> and although linux works with minimum section 128Mb but Windows memory
> hotplug works just fine even with page-sized slots so when unplug in
> QEMU is implemented it would be possible to drop balooning driver at
> least there.

I think for a big (64G?) guest it doesn't make much sense anyway to
balloon at a granularity that is less than 1G or even more.  So I like
the idea of dropping ballooning in favor of memory hotplug for big guests.

> And providing that memslots could be allocated during runtime when guest
> programs devices or maps roms (i.e. no fail path), I don't see a way
> to fix it in QEMU (i.e. avoid abort when limit is reached).
> Hence an attempt to bump memslots limit to 512, where current 125
> are reserved for initial memory mappings and passthrough devices 
> 256 goes to hotplug memory slots and leaves us 128 free slots for
> future expansion.
> 
> To see what would be affected by large amount of slots I played with
> perf a bit and the biggest hotspot offender with large amount of
> memslots was:
> 
>  gfn_to_memslot() -> ... -> search_memslots()
> 
> I'll try to make it faster for this case so 512 memslots wouldn't
> affect guest performance.
> 
> So please consider applying this patch.

Yes, sorry for the delay---I am definitely going to apply it.

Paolo