On 22/04/2022 15.52, Hao Xu wrote:
Hi Avi,
在 4/13/22 6:33 PM, Avi Kivity 写道:
Unfortunately, only ideas, no patches. But at least the first seems
very easy.
- IORING_OP_MEMCPY_IMMEDIATE - copy some payload included in the op
itself (1-8 bytes) to a user memory location specified by the op.
Linked to another op, this can generate an in-memory notification
useful for busy-waiters or the UMWAIT instruction
This would be useful for Seastar, which looks at a timer-managed
memory location to check when to break computation loops.
- IORING_OP_MEMCPY - asynchronously copy memory
Some CPUs include a DMA engine, and io_uring is a perfect interface
to exercise it. It may be difficult to find space for two iovecs though.
I have a question about the 'DMA' here, do you mean DMA device for
memory copy?
Yes. I understand some Intel server processors have them.
My understanding is you want async memcpy so that the
cpu can relax when the specific hardware is doing memory copy. the
thing is for cases like busy waiting or UMAIT, the length of the memory
to be copied is usually small(otherwise we don't use busy waiting or
UMAIT, right?). Then making it async by io_uring's iowq may introduce
much more overhead(the context switch).
These are two separate cases.
1. Bulk data copies (usually large), use DMA
2. small memory writes to wake up a thread that is using UMWAIT or
busy-polling, do not use DMA
Whether to use DMA or not can be based on writes size. I'd say anything
<= 512 bytes should be done by the CPU.
