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

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Date:	Thu, 06 Nov 2014 17:23:58 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>, linux-kernel@...r.kernel.org
CC:	kvm@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] kvm: x86: increase user memory slots to 509

On 06/11/2014 16:52, Igor Mammedov wrote:
> With the 3 private slots, this gives us 512 slots total.
> Motivation for this is in addition to assigned devices
> support more memory hotplug slots, where 1 slot is
> used by a hotplugged memory stick.
> It will allow to support upto 256 hotplug memory
> slots and leave 253 slots for assigned devices and
> other devices that use them.
> 
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>

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.

We probably should change the kmemdup + heap sort of
__kvm_set_memory_region + update_memslots to copy the array and insert
the new item at the right place, at the same time.  Using a heap sort is
overkill and unnecessarily goes from O(n^2) to O(n^2 log n).  With a
bigger constant in front as well.

If you want to do it, I'd be grateful.  Otherwise I can look at it as
time permits.

Paolo
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