Hi Shally, Ahmed, and anyone else interested in compressdev,
I mentioned last week that we've been exploring using something other than
mbufs to pass src/dst buffers to compressdev PMDs.
Reasons:
- mbuf data is limited to 64k-1 in each segment of a chained mbuf. Data for
compression
can be greater and it would add cycles to have to break up into smaller
segments.
- data may originate in mbufs, but is more likely, particularly for storage
use-cases, to
originate in other data structures.
- There's a 2 cache-line overhead for every segment in a chain, most of this
data
is network-related, not needed by compressdev
So moving to a custom structure would minimise memory overhead, remove
restriction on 64k-1 size and give more flexibility if compressdev ever needs
any comp-specific meta-data.
We've come up with a compressdev-specific structure using the struct iovec from
sys/uio.h, which is commonly used by storage applications. This would replace
the src and dest mbufs in the op.
I'll not include the code here - Pablo will push that to github shortly and
we'd appreciate review comments there.
https://github.com/pablodelara/dpdk-draft-compressdev
Just posting on the mailing list to give a heads-up and ensure this reaches a
wider audience than may see it on github.
Note : We also considered having no data structures in the op, instead the
application
would supply a callback which the PMD would use to retrieve meta-data (virt
address, iova, length)
for each next segment as needed. While this is quite flexible and allow the
application
to keep its data in its native structures, it's likely to cost more cycles.
So we're not proposing this at the moment, but hope to benchmark it later while
the API is still experimental.
General feedback on direction is welcome here on the mailing list.
For feedback on the details of implementation we would appreciate comments on
github.
Regards,
Fiona.
