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You and the parent are both "missing the point", which is sadly not talked about by the manufacturer either (IBM). I used to work for Citrix, which is "softwar
by jiggawatts 2y ago
You and the parent are both "missing the point", which is sadly not talked about by the manufacturer either (IBM).
I used to work for Citrix, which is "software that turns Windows into a mainframe OS". Basically, you get remote thin terminals the same as you would with an IBM mainframe, but instead of showing you green text you get a Windows desktop.
Citrix used to sell this as a "cost saving" solution that inevitably would cost 2-3x the same as traditional desktops.
The real benefit for both IBM mainframes and Citrix is: latency.
You can't avoid the speed of light, but centralising data and compute into "one box" or as close as you can get it (one rack, one data centre, etc...) provides enormous benefits to most kinds of applications.
If you have some complex business workflow that needs to talk to dozens of tables in multiple logical databases, then having all of that unfold in a single mainframe will be faster than if it has to bounce around a network in a "modern" architecture.
In real enterprise environments (i.e.: not a FAANG) any traffic that has to traverse between servers will typically use 10 Gbps NICs at best (not 100 Gbps!), have no topology optimisation of any kind, and flow through at a minimum one load balancer, one firewall, one router, and multiple switches.
Within a mainframe you might have low double-digit microsecond latencies between processes or LPARs, across an enterprise network between services and independent servers its not unusual to get well over one millisecond -- one hundred times slower.
This is why mainframes are still king for many orgs: They're the ultimate solution for dealing with speed-of-light delays.
PS: I've seen multiple attempts to convert mainframe solutions to modern "racks of boxes" and it was hilarious to watch the architects be totally mystified as to why everything was running like slow treacle when on paper the total compute throughput was an order of magnitude higher than the original mainframe had. They neglected latency in their performance modelling, that's why!
- le-mark 2y agoI’d love to read more about these projects. In particular, were they rewrites, or “rehosting”? What domain and what was the avg transaction count? Real-time or batch?
- jiggawatts 2y agoCitrix is almost always used to re-host existing applications. I've only ever seen very small utility apps that were purpose designed for Citrix and always as a part of a larger solution that mostly revolved around existing applications. Note that Citrix or any similar Windows "terminal services" or "virtual desktop" product fills the same niche as ordinary web applications, except that Win32 GUI apps are supported instead of requiring a rewrite to HTML. The entire point is that existing apps can be hosted with the same kind of properties as a web app, minus the rewrite.
- le-mark 2y agoI was referring to mainframe migrations, sorry that wasn’t clear.
- jiggawatts 2y agoI watched two such mainframe to “modern” architecture transitions recently, one at a telco and one at a medical insurance company. Both replaced what were billing and ERP systems. Both used Java on Linux virtual machines using an n-tier service oriented architecture. Both had a mix of batch and interactive modules. Both suffered from the same issue, which is actually very common but nobody seems to know: power efficiency throttling of CPU speeds. The irony was that the new compute platform had such a huge capacity compared to the old mainframe (20x or more) that the CPUs were only about 1% utilised. The default setting on all such servers is to turn cores off or put them into low-power modes as slow as 400 MHz. This murders performance and especially slows down the network because of the added latency of cores having to wake up from deep sleep when a packet arrives. It was one of those situations where running a busy-loop script on each server would speed up the application because it keeps everything “awake”. The telco doubled their capacity as an attempt to fix the issues but this took them to 0.5% utilisation and things got worse. The health insurer also overcomplicated their network, building a ~100 server cluster as if it was the public cloud. They had nested VLANs, address translation, software defined networking, firewalls between everything, etc… Latency was predictably atrocious and the whole thing ran like it was in slow motion. They too had the CPU throttling issue until I told them about it but the network was so bad it didn’t fix the overall issue.
- neverartful 2y agoThe mainframe itself (or any other platform for that matter) is not magical with regards to latency. It's all about proper architecture for the workload. Mainframes do provide a nice environment for being able to push huge volumes of IO though.
- jiggawatts 2y agoAgain, missing the point. Just look at the numbers. Mainframe manufacturers talk about "huge IO throughputs" but a rack of x86 kit with ordinary SSD SAN storage will have extra zeroes on the aggregate throughput. Similarly, on a bandwidth/dollar basis, Intel-compatible generic server boxes are vastly cheaper than any mainframe. Unless you're buying the very largest mainframes ($billions!), then for the same price a single Intel box will practically always win if you spend the same budget. E.g.: just pack it full of NVMe SSDs and enjoy ~100GB/s cached read throughput on top of ~20GB/s writes to remote "persistent" storage. The "architecture" here is all about the latency. Sure, you can "scale" a data centre full of thousands of boxes far past the maximums of any single mainframe, but then the latency necessarily goes up because of physics, not to mention the practicalities of large-scale Ethernet networking. The closest you can get to the properties of a mainframe is to put everything into one rack and use RDMA with Infiniband.
- throw4950sh06 2y ago> The closest you can get to the properties of a mainframe is to put everything into one rack and use RDMA with Infiniband. Or PCIe... I really would like to try building that.
- jiggawatts 2y agoI'm fairly certain you can't create a "mesh" with PCIe between multiple hosts. It's more like USB instead of Ethernet.
- throw4950sh06 2y agoDon't treat the CPU board as the host but a peripheral. ;-) But I'd say it's much closer to Ethernet than USB. You have controllers (routers), switches and nodes... USB doesn't, not like this.
- mmooss 2y ago> The real benefit for ... Citrix is: latency. I understand your point about big iron, but where does something like Citrix reduce latency? My estimation would be: Compared to a desktop Citrix adds distance and layers between the user and the compute/storage/etc., and the competition for resources on the Citrix server would tend to increase latency compared to the mostly idle desktop.
- Spooky23 2y agoThink WAN latency. Your shitty VB6 app using some awful circa 2002 middleware dies when you have 80ms of latency between you and the server. Insert Citrix, and the turd app is 3ms away in the data center and works. I used to run a 100k user VDI environment. The cost was easily 4x from a hardware POV, but I had 6 guys running it, and it was always consistent.
- mmooss 2y agoGood points, thanks. But please tell me you didn't have 100k people using that VB6 app with the middleware!