Return-Path: Received: from userp1040.oracle.com ([156.151.31.81]:47297 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750812AbbEARaa convert rfc822-to-8bit (ORCPT ); Fri, 1 May 2015 13:30:30 -0400 Content-Type: text/plain; charset=windows-1252 Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Subject: Re: [PATCH, RFC] backchannel overflows From: Chuck Lever In-Reply-To: <20150501172345.GB29624@infradead.org> Date: Fri, 1 May 2015 13:31:11 -0400 Cc: "J. Bruce Fields" , Linux NFS Mailing List , Trond Myklebust Message-Id: <32DAE15A-86A9-401A-B9E1-CBD9777D10CA@oracle.com> References: <1C0C92C2-FBCF-49D8-BB31-3C23A520B075@oracle.com> <20150429151404.GA12936@infradead.org> <20150429173454.GA23284@fieldses.org> <20150430062558.GA25660@infradead.org> <2ED34DBC-D928-4F1F-B5EF-B9F77D8AA075@oracle.com> <20150430143731.GA22038@infradead.org> <20CCFEFF-8D28-431C-A1E2-5E42FB42D8FB@oracle.com> <20150501172345.GB29624@infradead.org> To: Christoph Hellwig Sender: linux-nfs-owner@vger.kernel.org List-ID: On May 1, 2015, at 1:23 PM, Christoph Hellwig wrote: > On Thu, Apr 30, 2015 at 01:41:02PM -0400, Chuck Lever wrote: >> We discussed this briefly during the Linux NFS town hall meeting. >> I agree using dynamic slot allocation for TCP is fine, and RPC/RDMA >> can use simple overprovisioning. >> >> This way the upper layer (NFSv4.1 client) doesn?t have to be aware of >> limitations in the RPC layer mechanism. >> >> Trond may have an additional concern that I didn?t capture. > > The other option would be to simply overallocate in the transport layer, > as that is the layer which causes the problem to start with. That?s exactly what the RDMA backchannel will do. > That being said, what is the argument for doing any sort of static > allocation here? I'm fine with doing fully dynamic allocation if that > works out fine, but a mixed static / dynamic allocation sounds like a > nightmare. RDMA resources must be allocated and pre-registered up front. The RDMA transport can?t support dynamic slot allocation without adding a lot more complexity. The RDMA transport will need to have separate backchannel setup and destroy methods anyway. So doing dynamic for TCP and overprovision for RDMA will be simple. -- Chuck Lever chuck[dot]lever[at]oracle[dot]com