Re: [PATCH, RFC] rdma: split struct ib_send_wr

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

On Tue, Aug 04, 2015 at 08:06:16PM +0300, Sagi Grimberg wrote:
> Question though, a ULP may want to keep a couple of WRs around instead
> of having each allocated in the stack and handled one by one. We need
> to provide it with a hint of what is the size it needs.

Note that with the drastic shrink of the struct size the typical case
of needing a fast_reg + one send or a few rdma wr should be just fine
on stack now.  Even more so with your registration cleanups which
will drastically shrink the size of the fast reg mrs again.

> I just posted a patch to do that in iser 
> (http://www.spinics.net/lists/linux-rdma/msg27632.html).

That's actually part of the reason why I didn't manage to convert iser
as it seems so convoluted..

> So if I would want to preallocate an array of work requests, what is the
> size of the space I'd need?
> is it some form of max(sizeof(struct ib_send_wr),
>                        sizeof(struct ib_fastreg_wr),
>                        sizeof(struct sig_handover), ..)?

Preferably you'd preallocate them in a type safe manner, if you really
need to overlay them do it as a proper union.  For example the rds
code is doing that although it only preallocates on per work item.
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