On %D, %SN wrote: %Q %C Liang > -----Original Message----- > From: Michael S. Tsirkin [mailto:mst@xxxxxxxxxx] > Sent: Thursday, March 24, 2016 11:57 PM > To: Li, Liang Z > Cc: Dr. David Alan Gilbert; Wei Yang; qemu-devel@xxxxxxxxxx; > kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxx; pbonzini@xxxxxxxxxx; > rth@xxxxxxxxxxx; ehabkost@xxxxxxxxxx; amit.shah@xxxxxxxxxx; > quintela@xxxxxxxxxx; mohan_parthasarathy@xxxxxxx; > jitendra.kolhe@xxxxxxx; simhan@xxxxxxx; rkagan@xxxxxxxxxxxxx; > riel@xxxxxxxxxx > Subject: Re: [RFC Design Doc]Speed up live migration by skipping free pages > > On Thu, Mar 24, 2016 at 03:53:25PM +0000, Li, Liang Z wrote: > > > > > > Not very complex, we can implement like this: > > > > > > > > > > > > 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 2. > > > > > > Clear all the bits in ram_list. > > > > > > dirty_memory[DIRTY_MEMORY_MIGRATION] > > > > > > 3. Send the get_free_page_bitmap request 4. Start to send > > > > > > pages to destination and check if the free_page_bitmap is ready > > > > > > if (is_ready) { > > > > > > filter out the free pages from migration_bitmap_rcu->bmap; > > > > > > migration_bitmap_sync(); > > > > > > } > > > > > > continue until live migration complete. > > > > > > > > > > > > > > > > > > Is that right? > > > > > > > > > > The order I'm trying to understand is something like: > > > > > > > > > > a) Send the get_free_page_bitmap request > > > > > b) Start sending pages > > > > > c) Reach the end of memory > > > > > [ is_ready is false - guest hasn't made free map yet ] > > > > > d) normal migration_bitmap_sync() at end of first pass > > > > > e) Carry on sending dirty pages > > > > > f) is_ready is true > > > > > f.1) filter out free pages? > > > > > f.2) migration_bitmap_sync() > > > > > > > > > > It's f.1 I'm worried about. If the guest started generating the > > > > > free bitmap before (d), then a page marked as 'free' in f.1 > > > > > might have become dirty before (d) and so (f.2) doesn't set the > > > > > dirty again, and so we can't filter out pages in f.1. > > > > > > > > > > > > > As you described, the order is incorrect. > > > > > > > > Liang > > > > > > > > > So to make it safe, what is required is to make sure no free list us > > > outstanding before calling migration_bitmap_sync. > > > > > > If one is outstanding, filter out pages before calling > migration_bitmap_sync. > > > > > > Of course, if we just do it like we normally do with migration, then > > > by the time we call migration_bitmap_sync dirty bitmap is completely > > > empty, so there won't be anything to filter out. > > > > > > One way to address this is call migration_bitmap_sync in the IO > > > handler, while VCPU is stopped, then make sure to filter out pages > > > before the next migration_bitmap_sync. > > > > > > Another is to start filtering out pages upon IO handler, but make > > > sure to flush the queue before calling migration_bitmap_sync. > > > > > > > It's really complex, maybe we should switch to a simple start, just > > skip the free page in the ram bulk stage and make it asynchronous? > > > > Liang > > You mean like your patches do? No, blocking bulk migration until guest > response is basically a non-starter. > No, don't wait anymore. Like below (copy from previous thread) -------------------------------------------------------------- 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 2. Clear all the bits in ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION] 3. Send the get_free_page_bitmap request 4. Start to send pages to destination and check if the free_page_bitmap is ready if (is_ready) { filter out the free pages from migration_bitmap_rcu->bmap; migration_bitmap_sync(); } continue until live migration complete. --------------------------------------------------------------- Can this work? Liang > -- > MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
