linux-kernel - Re: [PATCH] RDMA/core: reduce IB_POLL

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Date:   Tue, 20 Feb 2018 16:47:31 -0500
From:   Chuck Lever <chuck.lever@...cle.com>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "jgg@...pe.ca" <jgg@...pe.ca>, "arnd@...db.de" <arnd@...db.de>,
        "dledford@...hat.com" <dledford@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "leonro@...lanox.com" <leonro@...lanox.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "sagi@...mberg.me" <sagi@...mberg.me>
Subject: Re: [PATCH] RDMA/core: reduce IB_POLL_BATCH constant



> On Feb 20, 2018, at 4:14 PM, Bart Van Assche <Bart.VanAssche@....com> 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