Re: [Qemu-devel] [PATCH 1/1] Count used RAMBlock pages for migration_dirty_pages

[Juan Quintela]

Paolo Bonzini <address@hidden> wrote:
> Il 21/03/2014 16:45, Dr. David Alan Gilbert ha scritto:
>>> > So it looks like your patch could also fix the problem Juan reported
>>> > at http://article.gmane.org/gmane.comp.emulators.qemu/247462 -- but
>>> > perhaps only on hosts where !KVM || TARGET_PAGE_SIZE==getpagesize().
>> Possibly yes; I think that would cause gaps in the bitmap to have
>> the same effect as the hot unplug.
>
> It would be nice to revive that patch, because without it you risk
> getting bad performance from migration of hotplugged memory.  On-board
> RAM typically gets low ram_addr_t's that are aligned, but the 128K (32
> pages) ROM and the 64K VGA BIOS will ruin the alignment and cause
> migration to use the slow paths to migrate hotplugged memory above
> them.

Agreed.  I had forgot that optimization that we had to get out.

The problem (for my use case) was VGA, that was dirtying continously
memory that was not aligned.  Size was not big, but was using bit-by-bit
operations, when we could use bitmap ones.

Later, Juan.