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Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr
Date: Mon, 12 May 2014 10:16:21 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0

Am 12.05.2014 08:09, schrieb Paolo Bonzini:
> Il 10/05/2014 18:32, Peter Lieven ha scritto:
>> What about XEN?
> You're right, Xen wouldn't work.  Your original patch would not break it just 
> because Xen doesn't use migration (but the code would be broken).
> You would have to cache qemu_get_ram_block rather than qemu_get_ram_ptr, move 
> RAMBlock to memory-internal.h, and split the RAMBlock + ram_addr_t => void * 
> conversion out of qemu_get_ram_ptr and into a separate function (to be used 
> by memory_region_get_ram_ptr).
> I'm not sure of the benefit of your patch though.  qemu_get_ram_block already 
> has a 1-item cache, are you seeing a low hit rate there?  Or any other 
> profiling that shows qemu_get_ram_ptr as hot?

qemu_get_ram_ptr is hot only during migration. But the hit-rate of the LRU 
cache seems to be good. I am wondering if this is different if the migration
has difficulties to converge, but you might be right it should be neglectible.

I ran some basic migration tests with and without the patch. It might be that 
the results with the cache are slightly better, but the variance of the
results is high. I had to run a significant number of tests to get more 


> Paolo

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