linux-kernel - Re: [PATCH RFC] vhost: convert pre sorted vhost memory array to interval tree

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Message-ID: <569EF607.2070107@redhat.com>
Date:	Wed, 20 Jan 2016 10:50:47 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	mst@...hat.com, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] vhost: convert pre sorted vhost memory array to
 interval tree



On 01/19/2016 11:24 PM, Igor Mammedov wrote:
> On Mon, 18 Jan 2016 10:42:29 +0800
> Jason Wang <jasowang@...hat.com> wrote:
>
>> > Current pre-sorted memory region array has some limitations for future
>> > device IOTLB conversion:
>> > 
>> > 1) need extra work for adding and removing a single region, and it's
>> >    expected to be slow because of sorting or memory re-allocation.
>> > 2) need extra work of removing a large range which may intersect
>> >    several regions with different size.
>> > 3) need trick for a replacement policy like LRU
>> > 
>> > To overcome the above shortcomings, this patch convert it to interval
>> > tree which can easily address the above issue with almost no extra
>> > work.
>> > 
>> > The patch could be used for:
>> > 
>> > - Extend the current API and only let the userspace to send diffs of
>> >   memory table.
>> > - Simplify Device IOTLB implementation.
> I'm curios how performance changes on translate_desc() hot-path in
> default case and in the case of 256 memory regions?
>

Haven't measured this. But consider both methods are O(logN), should be
no noticeable difference. Will measure it in the future.

The main user is for the device IOTLB API[1], which:

- have thousands of userspace memory region sections
- may dynamically adding or removing one or some of the regions
- need a replacement algorithm like LRU

[1] https://lkml.org/lkml/2015/12/31/16