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Thu, 02 Sep 2021 03:25:35 -0700 (PDT) X-Google-Smtp-Source: ABdhPJzxjpN6Jx7EesXpxLMghTm/E7SO8kVPY7G4JMPH2Hovs3cL+XhB4T0tFSTI+nyvVZD6vIJrv5HSRKCt0iUaLK8= X-Received: by 2002:a05:651c:10a2:: with SMTP id k2mr1915190ljn.262.1630578334701; Thu, 02 Sep 2021 03:25:34 -0700 (PDT) MIME-Version: 1.0 References: <20210831110238.299458-1-leobras@redhat.com> <20210831110238.299458-4-leobras@redhat.com> In-Reply-To: From: Leonardo Bras Soares Passos Date: Thu, 2 Sep 2021 07:25:46 -0300 Message-ID: Subject: Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy To: =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=lsoaresp@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=216.205.24.124; envelope-from=lsoaresp@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.392, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Elena Ufimtseva , John G Johnson , Jagannathan Raman , qemu-block@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , Peter Xu , qemu-devel , Paolo Bonzini , =?UTF-8?B?TWFyYy1BbmRyw6kgTHVyZWF1?= , Fam Zheng Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Sep 2, 2021 at 6:59 AM Daniel P. Berrang=C3=A9 wrote: > > On Thu, Sep 02, 2021 at 06:49:06AM -0300, Leonardo Bras Soares Passos wro= te: > > On Thu, Sep 2, 2021 at 6:20 AM Daniel P. Berrang=C3=A9 wrote: > > > > > > On Thu, Sep 02, 2021 at 05:52:15AM -0300, Leonardo Bras Soares Passos= wrote: > > > > On Thu, Sep 2, 2021 at 5:21 AM Daniel P. Berrang=C3=A9 wrote: > > > > > > > > > > On Thu, Sep 02, 2021 at 04:22:55AM -0300, Leonardo Bras Soares Pa= ssos wrote: > > > > > > Hello Daniel, thanks for the feedback ! > > > > > > > > > > > > On Tue, Aug 31, 2021 at 10:17 AM Daniel P. Berrang=C3=A9 wrote: > > > > > > > > > > > > > > On Tue, Aug 31, 2021 at 08:02:39AM -0300, Leonardo Bras wrote= : > > > > > > > > Call qio_channel_set_zerocopy(true) in the start of every m= ultifd thread. > > > > > > > > > > > > > > > > Change the send_write() interface of multifd, allowing it t= o pass down > > > > > > > > flags for qio_channel_write*(). > > > > > > > > > > > > > > > > Pass down MSG_ZEROCOPY flag for sending memory pages, while= keeping the > > > > > > > > other data being sent at the default copying approach. > > > > > > > > > > > > > > > > Signed-off-by: Leonardo Bras > > > > > > > > --- > > > > > > > > migration/multifd-zlib.c | 7 ++++--- > > > > > > > > migration/multifd-zstd.c | 7 ++++--- > > > > > > > > migration/multifd.c | 9 ++++++--- > > > > > > > > migration/multifd.h | 3 ++- > > > > > > > > 4 files changed, 16 insertions(+), 10 deletions(-) > > > > > > > > > > > > > > > @@ -675,7 +676,8 @@ static void *multifd_send_thread(void *= opaque) > > > > > > > > } > > > > > > > > > > > > > > > > if (used) { > > > > > > > > - ret =3D multifd_send_state->ops->send_writ= e(p, used, &local_err); > > > > > > > > + ret =3D multifd_send_state->ops->send_writ= e(p, used, MSG_ZEROCOPY, > > > > > > > > + = &local_err); > > > > > > > > > > > > > > I don't think it is valid to unconditionally enable this feat= ure due to the > > > > > > > resource usage implications > > > > > > > > > > > > > > https://www.kernel.org/doc/html/v5.4/networking/msg_zerocopy.= html > > > > > > > > > > > > > > "A zerocopy failure will return -1 with errno ENOBUFS. This= happens > > > > > > > if the socket option was not set, the socket exceeds its o= ptmem > > > > > > > limit or the user exceeds its ulimit on locked pages." > > > > > > > > > > > > You are correct, I unfortunately missed this part in the docs := ( > > > > > > > > > > > > > The limit on locked pages is something that looks very likely= to be > > > > > > > exceeded unless you happen to be running a QEMU config that a= lready > > > > > > > implies locked memory (eg PCI assignment) > > > > > > > > > > > > Do you mean the limit an user has on locking memory? > > > > > > > > > > Yes, by default limit QEMU sees will be something very small. > > > > > > > > > > > If so, that makes sense. I remember I needed to set the upper l= imit of locked > > > > > > memory for the user before using it, or adding a capability to = qemu before. > > > > > > > > > > > > Maybe an option would be trying to mlock all guest memory befor= e setting > > > > > > zerocopy=3Don in qemu code. If it fails, we can print an error = message and fall > > > > > > back to not using zerocopy (following the idea of a new io_asyn= c_writev() > > > > > > I told you in the previous mail). > > > > > > > > > > Currently ability to lock memory is something that has to be conf= igured > > > > > when QEMU starts, and it requires libvirt to grant suitable permi= ssions > > > > > to QEMU. Memory locking is generally undesirable because it preve= nts > > > > > memory overcommit. Or rather if you are allowing memory overcommi= t, then > > > > > allowing memory locking is a way to kill your entire host. > > > > > > > > You mean it's gonna consume too much memory, or something else? > > > > > > Essentially yes. > > > > Well, maybe we can check for available memory before doing that, > > but maybe it's too much effort. > > From a mgmt app POV, we assume QEMU is untrustworthy, so the mgmt > app needs to enforce policy based on the worst case that the > VM configuration allows for. > > Checking current available memory is not viable, because even > if you see 10 GB free, QEMU can't know if that free memory is > there to satisfy other VMs's worst case needs, or its own. QEMU > needs to be explicitly told whether its OK to use locked memory, > and an enforcement policy applied to it. Yeah, it makes sense to let the mgmt app deal with that and enable/disable the MSG_ZEROCOPY on migration whenever it sees fit. > > > > > Consider a VM host with 64 GB of RAM and 64 GB of swap, and an > > > overcommit ratio of 1.5. ie we'll run VMs with 64*1.5 GB of RAM > > > total. > > > > > > So we can run 3 VMs each with 32 GB of RAM, giving 96 GB of usage > > > which exceeds physical RAM. Most of the time this may well be fine > > > as the VMs don't concurrently need their full RAM allocation, and > > > worst case they'll get pushed to swap as the kernel re-shares > > > memory in respose to load. So perhaps each VM only needs 20 GB > > > resident at any time, but over time one VM can burst upto 32 GB > > > and then 12 GB of it get swapped out later when inactive. > > > > > > But now consider if we allowed 2 of the VMs to lock memory for > > > purposes of migration. Those 2 VMs can now pin 64 GB of memory > > > in the worst case, leaving no free memory for the 3rd VM or > > > for the OS. This will likely take down the entire host, regardless > > > of swap availability. > > > > > > IOW, if you are overcomitting RAM you have to be extremely > > > careful about allowing any VM to lock memory. If you do decide > > > to allow memory locking, you need to make sure that the worst > > > case locked memory amount still leaves enough unlocked memory > > > for the OS to be able to effectively manage the overcommit > > > load via swap. We definitely can't grant memory locking to > > > VMs at startup in this scenario, and if we grant it at runtime, > > > we need to be able to revoke it again later. > > > > > > These overcommit numbers are a bit more extreme that you'd > > > usually do in real world, but it illustrates the genreal > > > problem. Also bear in mind that QEMU has memory overhead > > > beyond the guest RAM block, which varies over time, making > > > accounting quite hard. We have to also assume that QEMU > > > could have been compromised by a guest breakout, so we > > > can't assume that migration will play nice - we have to > > > assume the worst case possible, given the process ulimits. > > > > > > > Yeah, that makes sense. Thanks for this illustration and elucidation ! > > > > I assume there is no way of asking the OS to lock memory, and if there = is > > no space available, it fails and rolls back the locking. > > Yes & no. On most Linux configs though it ends up being no, because > instead of getting a nice failure, when host memory pressure occurs, > the OOM Killer wakes up and just culls processes. oh, right the OOM Killer :) > > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberran= ge :| > |: https://libvirt.org -o- https://fstop138.berrange.c= om :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberran= ge :| > Thanks! Best regards, Leonardo