zeeshanul huq - Sun Microsystems - Beijing China wrote:
Hi Brian,
The overhead of it is not only dma binding, but also unbinding.
If no copybuf is used, the overhead of the unbinding is quite quite
small comparing to the binding.
And some other shortages are:
1) we have to hold the MBLKs until packet transmition complete. With
bcopy we are able to free them immediately. So when the system are
near to running out of MBLKs, bcopy works better.
I don't know when running out of MBLKs occurs. When the system is short
of kernel memory? If it is the case, then the extra bcopy also consumes
kernel memory.
2) In some driver like bge, it only has a small number of TX buffer
descriptor. With bcopy, it ensures one BD per transmit packet, while
it may require more than one with dma_bind. so using dma bind, it will
run out of Tx BD more quicker during heavy traffic.
Yes. This is reasonable.
That's part of the reasons why we use both bcopy and dma_bind in our
NIC driver. I agree we need a more faster dma binding and unbinding
solution.
What I suggested is one way to get much faster dma binding.
Of course, the original binding is also kept to meet the bcopy requirement.
Thanks,
Brian
Regards,
Zeeshanul Huq
Garrett D'Amore wrote:
Brian Xu - Sun Microsystems - Beijing China wrote:
Hi there,
I have a question here:
Why all of the NIC drivers have to bcopy the MBLKs for transmit?
(some of them bcopy always, and some others bcopy under a threshold
of the packet length).
I think one of the reason is the overhead of the setup of dma on the
fly is greater than the overhead of bcopy for short packets. I want
to know if this is the case and if there are any other reasons.
Yes. For any packet reasonably sized bcopy (ETHERMTU or smaller) is
faster on *all* recent hardware. (This is confirmed on even an older
300MHz Via C3.) (Hmm... I've heard that for some Niagra systems
this might not be true, however. But I've not tested it myself.)
I think the situation is different with jumbo frames, though.
If what I guess is the major cause, I have a proposal and I want to
hear your advice whether it makes sense.
The most time-consuming action for the dma setup is the dma bind,
more specific, calling into the VM layer to get the PFN for the
vaddr(hat_getpfnum()), since it need to search the huge page table.
While for the MBLKs, essentially which are slab objects, the PFN
has already been determined in the slab layer, and for most of their
usage, we only touch the magazine layer, where the PFN is a pre
determined one. That is, the PFN should be considered as a
constructed state, but we don't leverage it for dma bind.
In storage, we have a field 'b_shadow' in buf(9S) to store the
pages which are recently used, through which the PFNs can be easily
got. so in
the case that b_shadow works, ddi_dma_buf_bind_handle() is much
faster than the ddi_dma_mem_bind_handle().
Another example, moving the dma bind of the HBA driver(mpt) from Tx
path to the kmem cache constrcutor, mpt driver got 26% throughput
increment. See CR6707308.
If the mblk could store the PFN info and we had a
ddi_dma_mblk_bind_handle() like interface, then I think it will
benefit the performance of the NIC drivers. I consulted the PAE,
and got a answer that the bcopy is typically about 10-15% of a NIC
TX workload.
There are things that can do to make DMA faster, better, and
simpler. In an ideal world, the GLDv3 could do most of this work,
and the mblk could just carry the ddi_dma_cookie with it.
-- Garrett
Thanks,
Brian
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