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Re: Grant operation batching
- To: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 20 May 2022 14:44:45 +0200
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- Cc: Xen developer discussion <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 20 May 2022 12:44:59 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 20.05.2022 14:37, Demi Marie Obenour wrote:
> On Fri, May 20, 2022 at 08:24:48AM +0200, Jan Beulich wrote:
>> On 20.05.2022 01:22, Demi Marie Obenour wrote:
>>> It is well known that mapping and unmapping grants is expensive, which
>>> is why blkback has persistent grants. Could this cost be mitigated by
>>> batching, and if it was, would it affect the tradeoff of memcpy() vs
>>> grant table operations?
>>
>> Which backend driver are you thinking about? The in-kernel Linux
>> xen-blkback already batches grant operations, afaics. Such
>> batching is helpful, but the main cost is assumed (known?) to be
>> with the (installing and) tearing down of the actual mappings of
>> the guest pages (into/)from backend address space.
>
> My thought was that the expensive part of this is TLB flushes, which are
> only needed once per batch.
Correct, but as said - such batching is already being made use of by
the in-kernel Linux backend. Of course "once per batch" is not entirely
precise - very large batches would be split internally into smaller
chunks, each of which would be followed by a TLB flush.
> Also, what do you think about the “unsafe”
> mode? It would only be unsafe if the backend is untrusted, but it is
> quite common for the backend to be trusted.
Well, I didn't put much thought into that (and hence intentionally
didn't comment on it), so for now it's only "Why not?"
Jan
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