PS. That last paragraph was intended to respond to John not Nico.
On Sun, Aug 24, 2014 at 3:27 PM, Paul Robert Marino <prmari...@gmail.com> wrote: > Nico > Depending on the role of the particular system and or which company I > was working for at the time I've need one the other or both. > In my current role in the broadcast industry precision with > predictable latency is more important for most of my systems. > That said when I worked in the financial industry it changed based on > what part of the industy I was working for. > The stock exchanges I've worked for cared about precision because it > was more important to them to make sure every one had the same latency > and our logging was accurate for audit purposes. > When I used to work for a managed systems vender for hedge funds they > were all about low latency because the faster they got data in and out > of the exchanges often determined if they had an edge over their > competitors or not. quite literally an extra millisecond could cost > them millions of dollars a second due to the nature of high frequency > trading. > > Generally I don't think of real time kernels when I am thinking about > low latency because oftent they increase the latency when dealing with > multiple operations. however the reverse can true if you only have a > box doing one specific task only but that rarely is the case. > > By the way one of those stock exchanges is where the VMware engineers > told us never to use their product in production. In fact we had huge > problems with VMware in our development environments because some of > our applications would actually detect the clock instability in the > VMware clocks and would shut themselves down rather than have > inaccurate audit logs. as a result we found we had trouble even using > it in our development environments. > > By the way Red hat only told me recently about guaranteeing the > microsecond precision of the clocks in KVM on RHEV and said they have > been doing it in financial for over a year. there are conditions > though such as you need to turn off support for overbooking the CPU > cores. last I checked VMware still says do not use their product > anywhere where you need millisecond accurate clocks. > > Further more I dont know about that statement "Anyways, KVM will not > handle latency any better than Vmware." the article you pointed out > talks about VCPU's going in and out of halted states, which is normal > and completely expected in VMware because they always assume you are > going to overbook your CPU cores. there is a slight difference when > you talk about KVM in paravirtualized mode with overbooking disabled > it directly maps the CPU cores the the VM so as long as you don't have > power management enabled the CPU's are always operating at full speed > further more you can directly map PCIe bus address to the VM > (essentially assigning a card on your bus directly to the VM to be > completely managed by its kernel) to reduce latency in other ways to > hardware if you need too. > > On Sun, Aug 24, 2014 at 2:02 PM, Nico Kadel-Garcia <nka...@gmail.com> wrote: >> On Sun, Aug 24, 2014 at 12:57 PM, John Lauro >> <john.la...@covenanteyes.com> wrote: >>> Why spread FUD about Vmware. Anyways, to hear what they say on the subject: >>> http://www.vmware.com/files/pdf/techpaper/latency-sensitive-perf-vsphere55.pdf >>> >>> Anyways, KVM will not handle latency any better than Vmware. >>> >>> ----- Original Message ----- >>>> From: "Paul Robert Marino" <prmari...@gmail.com> >>>> To: "Nico Kadel-Garcia" <nka...@gmail.com> >>>> Cc: "Brandon Vincent" <brandon.vinc...@asu.edu>, llwa...@gmail.com, >>>> "SCIENTIFIC-LINUX-USERS@FNAL.GOV" >>>> <scientific-linux-users@fnal.gov> >>>> Sent: Sunday, August 24, 2014 12:26:17 PM >>>> Subject: Re: about realtime system >>>> >>>> Wow I don't know how VMware got mentioned in this string but VMware >>>> is >>>> not capable of real time operation and if you ask the senior >>>> engineers >>>> at VMware they will tell you they don't want you even trying it on >>>> their product because they know it wont work. The reason is VMware >>>> plays games with the clock on the VM so the clocks can never be 100% >>>> accurate. >>>> It should be possible to do real time in KVM assuming you don't >>>> overbook your CPU Cores or RAM. Apparently Red Hat has been doing >>>> VM's >>>> with microsecond accurate clocks with PTP running on the >>>> visualization >> >> >> I mentioned that I hope they were using real servers, not VM's. I'd >> had people try to run "real-time" systems in virtualization, >> specifically with VMware, and it wasn't workable for their needs. >> >> Also, "high precision" is not the same as "low latency", although both >> are often grouped together for "real-time" operations. I'm curious if >> Paul needs both.