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Subject: Re: [PATCH] RDMA/core: reduce IB_POLL_BATCH constant
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <1519161268.3737.12.camel@wdc.com>
Date:   Tue, 20 Feb 2018 16:47:31 -0500
Cc:     "jgg@ziepe.ca" <jgg@ziepe.ca>, "arnd@arndb.de" <arnd@arndb.de>,
        "dledford@redhat.com" <dledford@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "leonro@mellanox.com" <leonro@mellanox.com>,
        "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
        "sagi@grimberg.me" <sagi@grimberg.me>
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References: <20180220205924.2035765-1-arnd@arndb.de>
 <1519161268.3737.12.camel@wdc.com>
To:     Bart Van Assche <Bart.VanAssche@wdc.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Feb 20, 2018, at 4:14 PM, Bart Van Assche <Bart.VanAssche@wdc.com> =
wrote:
>=20
> 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
>=20
> 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