Re: [Qemu-devel] [Qemu-block] [PATCH] qcow2: do lazy allocation of the L2 cache

[Alberto Garcia]

On Fri 24 Apr 2015 11:52:14 AM CEST, Kevin Wolf <address@hidden> wrote:

>> The posix_memalign() call wastes memory.  I compared:
>> 
>>   posix_memalign(&memptr, 65536, 2560 * 65536);
>>   memset(memptr, 0, 2560 * 65536);
>> 
>> with:
>> 
>>   for (i = 0; i < 2560; i++) {
>>       posix_memalign(&memptr, 65536, 65536);
>>       memset(memptr, 0, 65536);
>>   }
>
> 64k alignment is too much, in practice you need 512b or 4k, which
> probably wastes a lot less memory.

My tests were with 512b and 4k and the overhead was around 4k per page
(hence 2560 * 4 = 10240, the 10MB I was talking about).

> But I just looked at the qcow2 cache code and you're right anyway.
> Allocating one big block instead of many small allocations in a loop
> looks like a good idea either way.

The problem is that I had the idea to make the cache dynamic.

Consider the scenario [A] <- [B], with a virtual size of 1TB and [B] a
newly created snapshot. The L2 cache size is 128MB for each image, you
read a lot of data from the disk and the cache from [A] starts to fill
up (at this point [B] is mostly empty so you get all the data from
[A]). Then you start to write data into [B], and now its L2 cache starts
to fill up as well.

After a while you're going to have lots of cache entries in [A] that are
not needed anymore because now the data for those clusters is in [B].

I think it would be nice to have a way to free unused cache entries
after a while.

Berto