Re: [PATCH] RDMA/core: reduce IB_POLL_BATCH constant

[Chuck Lever]

> On Feb 20, 2018, at 4:14 PM, Bart Van Assche <Bart.VanAssche@xxxxxxx> wrote:
> 
> On Tue, 2018-02-20 at 21:59 +0100, Arnd Bergmann wrote:
>> /* # of WCs to poll for with a single call to ib_poll_cq */
>> -#define IB_POLL_BATCH			16
>> +#define IB_POLL_BATCH			8
> 
> The purpose of batch polling is to minimize contention on the cq spinlock.
> Reducing the IB_POLL_BATCH constant may affect performance negatively. Has
> the performance impact of this change been verified for all affected drivers
> (ib_srp, ib_srpt, ib_iser, ib_isert, NVMeOF, NVMeOF target, SMB Direct, NFS
> over RDMA, ...)?

Only the users of the DIRECT polling method use an on-stack
array of ib_wc's. This is only the SRP drivers.

The other two modes have use of a dynamically allocated array
of ib_wc's that hangs off the ib_cq. These shouldn't need any
reduction in the size of this array, and they are the common
case.

IMO a better solution would be to change ib_process_cq_direct
to use a smaller on-stack array, and leave IB_POLL_BATCH alone.

--
Chuck Lever