From mboxrd@z Thu Jan 1 00:00:00 1970 From: Maxime Coquelin Subject: Re: [PATCH] eal: add option to not store segment fd's Date: Fri, 29 Mar 2019 14:35:58 +0100 Message-ID: References: <07f664c33ddedaa5dcfe82ecb97d931e68b7e33a.1550855529.git.anatoly.burakov@intel.com> <1682850.JO3elT0QtZ@xps> <3255576.YcZt162MTL@xps> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: David Marchand , dev , John McNamara , Marko Kovacevic , iain.barker@oracle.com, edwin.leung@oracle.com To: "Burakov, Anatoly" , Thomas Monjalon Return-path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by dpdk.org (Postfix) with ESMTP id 6B76A2BF4 for ; Fri, 29 Mar 2019 14:36:12 +0100 (CET) In-Reply-To: Content-Language: en-US List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On 3/29/19 2:24 PM, Burakov, Anatoly wrote: > On 29-Mar-19 12:40 PM, Thomas Monjalon wrote: >> 29/03/2019 13:05, Burakov, Anatoly: >>> On 29-Mar-19 11:34 AM, Thomas Monjalon wrote: >>>> 29/03/2019 11:33, Burakov, Anatoly: >>>>> On 29-Mar-19 9:50 AM, David Marchand wrote: >>>>>> On Fri, Feb 22, 2019 at 6:12 PM Anatoly Burakov >>>>>> > wrote: >>>>>> >>>>>>       Due to internal glibc limitations [1], DPDK may exhaust >>>>>> internal >>>>>>       file descriptor limits when using smaller page sizes, which >>>>>> results >>>>>>       in inability to use system calls such as select() by user >>>>>>       applications. >>>>>> >>>>>>       While the problem can be worked around using >>>>>> --single-file-segments >>>>>>       option, it does not work if --legacy-mem mode is also used. >>>>>> Add a >>>>>>       (yet another) EAL flag to disable storing fd's internally. This >>>>>>       will sacrifice compability with Virtio with vhost-backend, but >>>>>>       at least select() and friends will work. >>>>>> >>>>>>       [1] >>>>>> https://mails.dpdk.org/archives/dev/2019-February/124386.html >>>>>> >>>>>> >>>>>> Sorry, I am a bit lost and I never took the time to look in the new >>>>>> memory allocation system. >>>>>> This gives the impression that we are accumulating workarounds, >>>>>> between >>>>>> legacy-mem, single-file-segments, now no-seg-fds. >>>>> >>>>> Yep. I don't like this any more than you do, but i think there are >>>>> users >>>>> of all of these, so we can't just drop them willy-nilly. My great hope >>>>> was that by now everyone would move on to use VFIO so legacy mem >>>>> wouldn't be needed (the only reason it exists is to provide >>>>> compatibility for use cases where lots of IOVA-contiguous memory is >>>>> required, and VFIO cannot be used), but apparently that is too much to >>>>> ask :/ >>>>> >>>>>> >>>>>> Iiuc, everything revolves around the need for per page locks. >>>>>> Can you summarize why we need them? >>>>> >>>>> The short answer is multiprocess. We have to be able to map and unmap >>>>> pages individually, and for that we need to be sure that we can, in >>>>> fact, remove a page because no one else uses it. We also need to store >>>>> fd's because virtio with vhost-user backend needs them to work, >>>>> because >>>>> it relies on sharing memory between processes using fd's. I guess you mean virtio-user. Have you looked how Qemu does to share the guest memory with external process like vhost-user backend? It works quite well with 2MB pages, even with large VMs. >>>> >>>> It's a pity adding an option to workaround a limitation of a corner >>>> case. >>>> It adds complexity that we will have to support forever, >>>> and it's even not perfect because of vhost. >>>> >>>> Might there be another solution? >>>> >>> >>> If there is one, i'm all ears. I don't see any solutions aside from >>> adding limitations. >>> >>> For example, we could drop the single/multi file segments mode and just >>> make single file segments a default and the only available mode, but >>> this has certain risks because older kernels do not support fallocate() >>> on hugetlbfs. >>> >>> We could further draw a line in the sand, and say that, for example, >>> 19.11 (or 20.11) will not have legacy mem mode, and everyone should use >>> VFIO by now and if you don't it's your own fault. >>> >>> We could also cut down on the number of fd's we use in single-file >>> segments mode by not using locks and simply deleting pages in the >>> primary, but yanking out hugepages from under secondaries' feet makes me >>> feel uneasy, even if technically by the time that happens, they're not >>> supposed to be used anyway. This could mean that the patch is no longer >>> necessary because we don't use that many fd's any more. >> >> This last option is interesting. Is it realistic? >> > > I can do it in current release cycle, but i'm not sure if it's too late > to do such changes. I guess it's OK since the validation cycle is just > starting? I'll throw something together and see if it crashes and burns. > Reducing the number of FDs is really important IMHO, as the application using the DPDK library could also need several FDs for other purpose.