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Re: [PATCH v2 0/6] migration/ram: Optimize for virtio-mem via RamDiscard

From: David Hildenbrand
Subject: Re: [PATCH v2 0/6] migration/ram: Optimize for virtio-mem via RamDiscardManager
Date: Thu, 29 Jul 2021 10:14:47 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

The thing is I still think this extra operation during sync() can be ignored by
simply clear dirty log during bitmap init, then.. why not? :)

I guess clearing the dirty log (especially in KVM) might be more expensive.

If we send one ioctl per cb that'll be expensive for sure.  I think it'll be
fine if we send one clear ioctl to kvm, summarizing the whole bitmap to clear.

The other thing is imho having overhead during bitmap init is always better
than having that during sync(). :)

Oh, right, so you're saying, after we set the dirty bmap to all ones and
excluded the discarded parts, setting the respective bits to 0, we simply
issue clearing of the whole area?

For now I assumed we would have to clear per cb.

Hmm when I replied I thought we can pass in a bitmap to ->log_clear() but I
just remembered memory API actually hides the bitmap interface..

Reset the whole region works, but it'll slow down migration starts, more
importantly that'll be with mmu write lock so we will lose most clear-log
benefit for the initial round of migration and stuck the guest #pf at the

Let's try do that in cb()s as you mentioned; I think that'll still be okay,
because so far the clear log block size is much larger (1gb), 1tb is worst case
1000 ioctls during bitmap init, slightly better than 250k calls during sync(),
maybe? :)

Just to get it right, what you propose is calling migration_clear_memory_region_dirty_bitmap_range() from each cb(). Due to the clear_bmap, we will end up clearing each chunk (e.g., 1GB) at most once.

But if our layout is fragmented, we can actually end up clearing all chunks (1024 ioctls for 1TB), resulting in a slower migration start.

Any gut feeling how much slower migration start could be with largish (e.g., 1 TiB) regions?


David / dhildenb

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