[Gluster-devel] Suggestion needed to make use of iobuf_pool as rdma buffer.

[Mohammed Rafi K C]

Hi All,

When using RDMA protocol, we need to register the buffer which is going
to send through rdma with rdma device. In fact, it is a costly
operation, and a performance killer if it happened in I/O path. So our
current plan is to register pre-allocated iobuf_arenas from  iobuf_pool
with rdma when rdma is getting initialized. The problem comes when all
the iobufs are exhausted, then we need to dynamically allocate new
arenas from libglusterfs module. Since it is created in libglusterfs, we
can't make a call to rdma from libglusterfs. So we will force to
register each of the iobufs from the newly created arenas with rdma in
I/O path. If io-cache is turned on in client stack, then all the
pre-registred arenas will use by io-cache as cache buffer. so we have to
do the registration in rdma for each i/o call for every iobufs,
eventually we cannot make use of pre registered arenas.

To address the issue, we have two approaches in mind,

 1) Register each dynamically created buffers in iobuf by bringing
transport layer together with libglusterfs.

 2) create a separate buffer for caching and offload the data from the
read response to the cache buffer in background.

If we could make use of preregister memory for every rdma call, then we
will have approximately 20% increment for write and 25% of increment for
read.

Please give your thoughts to address the issue.

Thanks & Regards
Rafi KC