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Re: [Qemu-devel] DIRTY_MEMORY_BLOCK_SIZE;
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] DIRTY_MEMORY_BLOCK_SIZE; |
Date: |
Wed, 5 Jul 2017 17:06:41 +0100 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
* Stefan Hajnoczi (address@hidden) wrote:
> On Fri, Jun 30, 2017 at 12:26:10PM +0100, Dr. David Alan Gilbert wrote:
> > * ali saeedi (address@hidden) wrote:
> > > Hello
> > > what does 'DIRTY_MEMORY_BLOCK_SIZE' mean?
> > > is it the number of words in a block? or number of pages in a block? or
> > > number of bytes in a block?
> > > thanks a lot
> >
> > (cc'ing Stefan)
> > I think that DIRTY_MEMORY_BLOCK_SIZE is the number of TARGET_PAGEs
> > within one DIRTY_MEMORY_BLOCK
> > So with the common 4k target page that's 4k*256k*8=8GB/dirty memory
> > block - note these are just the size of structure sin qemu, it's still
> > got the ranularity ot mark individual target pages as dirty.
>
> Right, the calculation is shown in the comment above the code:
>
> * rcu_read_lock();
> *
> * DirtyMemoryBlocks *blocks =
> * atomic_rcu_read(&ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]);
> *
> * ram_addr_t idx = (addr >> TARGET_PAGE_BITS) / DIRTY_MEMORY_BLOCK_SIZE;
> * unsigned long *block = blocks.blocks[idx];
> * ...access block bitmap...
> *
> * rcu_read_unlock();
>
> Rather than focussing on DIRTY_MEMORY_BLOCK_SIZE, make sure you
> understand how DirtyMemoryBlocks works. It is an array of bitmap
> pointers.
>
> Instead of directly indexing into a single huge dirty memory bitmap,
> QEMU divides the dirty memory bitmap into fixed-sized chunks. Each
> chunk covers DIRTY_MEMORY_BLOCK_SIZE pages.
>
> The reason for this layer of indirection is so that the dirty memory
> bitmap can be accessed without taking a traditional lock (just RCU) and
> also supports memory hotplug.
>
> Without indirection it would be difficult to grow the bitmap while other
> threads are writing to it. Thanks to the indirection, it's possible to
> allocate new chunks and continue using the old chunks when growth
> occurs.
I guess this works like the old non-chunk version, in that there's no
direct correspondence between DirtyMemoryBlocks and RAMBlock's - i.e.
one RAMBlock might span two DirtyMemoryBlocks even if it's smaller
than DIRTY_MEMORY_BLOCK_SIZE.
Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK