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Re: [RFC PATCH] xen/common: cache colored buddy allocator for domains
- To: Carlo Nonato <carlo.nonato@xxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 7 Dec 2022 12:52:23 +0100
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- Cc: marco.solieri@xxxxxxxxxx, andrea.bastoni@xxxxxxxxxxxxxxx, lucmiccio@xxxxxxxxx, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 07 Dec 2022 11:52:35 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.10.2022 18:08, Carlo Nonato wrote:
> This commit replaces the colored allocator for domains with a simple buddy
> allocator indexed also by colors, so that it can allocate pages based on
> some coloring configuration.
>
> It applies on top of Arm cache coloring (v3) as sent to the mailing list.
>
> This has two benefits:
> - order can now be greater than 0 if the color config contains a
> sufficient number of adjacent colors starting from an order aligned
> one;
But still not large enough to reach the order needed for large page
mappings, aiui?
> - same benefits of the normal buddy: constant time alloc and free
> (constant with respect to the number of pages, not for the number of
> colors);
>
> But also one "big" cons:
> - given the way Xen queries the allocator, it can only serve larger pages
> first and only when a domain runs out of those, it can go with the smaller
> ones. Let's say that domain 0 has 31 colors out of 32 total (0-30 out of
> 0-31). The order-4 pages (0-15) are allocated first and then the order-3
> (16-23, since 0-7 and 8-15 are all already allocated), and then order-2
> and so on. The result is... the domain practically uses only one half of
> the colors that it should.
What's unclear to me is how big of a con this is, i.e. how reasonable it is
for someone to configure a domain to use all except one of the colors (and
not, say, half of them).
Jan
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