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Re: [Qemu-devel] [PATCH 06/15] exec: use mmap for subpages

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 06/15] exec: use mmap for subpages
Date: Tue, 28 Jun 2016 12:48:15 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 28/06/2016 11:01, Peter Lieven wrote:
> a lot of subpages are created and freed at startup, but RCU delays
> the freeing so the heap gets fragmented.
> Signed-off-by: Peter Lieven <address@hidden>

I agree that subpages are bad for malloc because they are large (>
4KiB).  It is worth doing something special about them.  However, on
32-bit systems mmap-ing them has the same risk of fragmenting the
process address space, as malloc has of fragmenting the brk heap.

Allocation and freeing of subpages always happens under the BQL, so
perhaps a simple freelist is better?  Another interesting (but harder)
possibility could be to build the radix tree lazily.


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