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

[Sagi Grimberg <sagig-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>]

On 8/4/2015 8:23 PM, Christoph Hellwig wrote:
> 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 do agree that the size on the stack is less of an issue now. What
still can matter is handling each wr one by one vs. doing a collective
post.

>
>> 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..

I can understand that. In the last patches I tried to simplify the flow
(I noticed that these aren't in your tree correct?). There always room
for cleanups... I'm trying to have a gradual progress towards getting it
to be simpler.

>
>> 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.

Perhaps...
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