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From: Juan Quintela <quintela@redhat.com>
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	(Liang Z. Li's message of "Wed, 10 Aug 2016 05:04:31 +0000")
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Reply-To: quintela@redhat.com
Date: Wed, 10 Aug 2016 11:11:17 +0200
Message-ID: <877fbpouoa.fsf@emacs.mitica>
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Subject: Re: [Qemu-devel] [PATCH V2] add migration capability to bypass the
 shared memory
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To: "Li, Liang Z" <liang.z.li@intel.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Markus Armbruster <armbru@redhat.com>, Amit Shah <amit.shah@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>

"Li, Liang Z" <liang.z.li@intel.com> wrote:
>> On Wed, Aug 10, 2016 at 10:22 AM, Li, Liang Z <liang.z.li@intel.com> wrote:
>> > Hi Jiangshan,
>> >
>> > Glad to see your patch. It's a simple implementation which could provide
>> very useful functions.
>> >
>> >> +static void migration_bitmap_init(unsigned long *bitmap) {
>> >> +    RAMBlock *block;
>> >> +
>> >> +    bitmap_clear(bitmap, 0, last_ram_offset() >> TARGET_PAGE_BITS);
>> >> +    rcu_read_lock();
>> >> +    QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
>> >> +        if (!migrate_bypass_shared_memory()
>> >> || !qemu_ram_is_shared(block)) {
>> >> +            bitmap_set(bitmap, block->offset >> TARGET_PAGE_BITS,
>> >
>> > You should use (block->offset >> TARGET_PAGE_BITS )/ BITS_PER_LONG
>> here.
>> 
>> Hello, Li
>> 
>> I might have missed something, could you tell me more?
>> 
>> void bitmap_set(unsigned long *map, long start, long nr); I think the @start
>> and @nr are both the number of the bits.
>> 
>> thanks,
>> Lai
>
> You are right,  I have make a mistake by checking the code. Sorry for the noise.
>
> BTW. Is it possible to bypass the shared block in the 'ram_find_and_save_block'?
> I mean no to check if a page is dirty for a shared block, it may make things faster.

Nice spotted.  That would make things faster.  But once there we could
split the bitmap by ramblock, and the migration_dirty_pages also per
block, that would make all the searchs faster, and adding/removing
blocks of ram much easier.  If we enter optimizing that function, we
could really do much better that the "again" parameter that we have
right now.

Later, Juan.