Re: [PATCH rdma-core 1/2] verbs: Report the device's PCI write end paddding capability

[Jason Gunthorpe <jgg-uk2M96/98Pc@public.gmane.org>]

On Wed, Nov 15, 2017 at 03:10:30PM +0200, Yishai Hadas wrote:

> The man page was updated with a detailed description on the above
> capability, see PR:
> https://github.com/linux-rdma/rdma-core/pull/250

Yah, that is nicer

Here is some copy-editing:

Extended device capability flags (device_cap_flags_ex):
.br
.TP 7
IBV_DEVICE_PCI_WRITE_END_PADDING

Indicates the device has support for padding PCI writes to a full cache line.

Padding packets to full cache lines reduces the amount of traffic required at
the memory controller at the expense of creating more traffic on the PCI-E
port.

Workloads that have a high CPU memory load and low PCI-E utilization will
benefit from this feature, while workloads that have a high PCI-E utilization
and small packets will be harmed.

For instance, with a 128 byte cache line size, the transfer of any packets
less than 128 bytes will require a full 128 transfer on PCI, pontentially
doubling the required PCI-E bandwith.

This feature can be enabled on a QP or WQ basis via the
IBV_QP_CREATE_PCI_WRITE_END_PADDING or IBV_WQ_FLAGS_PCI_WRITE_END_PADDING
flags.
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