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How much CPU is there in a vCPU?
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How much CPU is there in a vCPU?

rchurchrchurch Member
edited September 2019 in Help

This vCPU issue has come up again for me.

How much compute power is there in a vCPU? I remember when Amazon stated in the early days of EC2 that a VM was equivalent to 1.7Ghz AMD Opteron or some other such chip.

There are all kinds of offers mentioning vCPUs here and there without stating exactly how much power is being offered.

Some people say a vCPU is equivalent to a thread or whatever?

Is the industry ever going to come out and standardize on what a vCPU is, relative to some known quantity, such as a Pentium D core clocked at 3.0Ghz?

I just noticed the thread about Oracle's Free Tier which mentions an OCPU, which I assume if Oracle's idiosyncratic measure of a vCPU.

Compute Classic - FAQ | Oracle Cloud Infrastructure

An OCPU is defined as the CPU capacity equivalent of one physical core of an Intel Xeon processor with hyper threading enabled. For Intel Xeon processor each OCPU corresponds to two hardware execution threads, known as vCPUs.

Is this the official definition of a vCPU?

When providers on LET talk about cores, do the mean one of these vCPUs, which is also being shared with other users?

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Comments

  • Depend

  • Yups for CPU with hyperthreading enabled vCPU equal to 0.5 CPU

  • bacloudbacloud Member, Patron Provider

    Basically almost all VPS are overselled and the most overselled thing in VPS are CPU. So vCPU is not even equal to a CPU thread.

    An example: We have product with not overselled resources and one real decent CPU core VPS costs 20 euro/month.

  • rchurch said: How much compute power is there in a vCPU?

    seven pounds of flax

    Thanked by 1dahartigan
  • An alternative question would be: How much vCPU is there in a CPU?

    Thanked by 3uptime Falzo ITLabs
  • Lets see how this planning goes:

    CPU we are taking into account is Intel Xeon CPU E5-2650 v2

    Intel Xeon CPU E5-2650 v2 holds 8 Cores  x8 = 64  vCPUs.
    4 vCPU to each VM…64 vCPUs / 4 vCPU per VM = 16 VMs
    2 vCPU to each VM…64 vCPUs / 2 vCPU per VM = 32 VMs
    1 vCPUs to each VM…64 vCPUs / 1 vCPU per VM = 64 VMs
    

    here is the detailed explanation:

  • Here's a useful video, keeping things simple:

  • raindog308raindog308 Administrator, Veteran

    uptime said: seven pounds of flax

    A monk asked Zen teacher Dòngshān (807–869): “What is the Buddha?”

    Dòngshān said, “Three pounds of flax.”

    -- traditional Zen koan

    Thanked by 1uptime
  • raindog308raindog308 Administrator, Veteran

    angstrom said: An alternative question would be: How much vCPU is there in a CPU?

    Depends if VM flags are enabled in BIOS.

    So the real question is "how much vCPU is there in a CPU if a CPU can do vCPU?"

  • The answer to all doubts is Hetzner

  • Just get a dedi.

    Thanked by 1JustPfff
  • jsgjsg Member, Resident Benchmarker

    Based on meanwhile quite some benchmarking I'd say a vCpu or vCore is typically between 15% and, if you are lucky, 50% of a hardware thread or, if you are really lucky, of a hw core. In the low end segment that is.

    Thanked by 3ITLabs JustPfff rchurch
  • jsgjsg Member, Resident Benchmarker

    @Sofia_K said:
    Lets see how this planning goes:

    CPU we are taking into account is Intel Xeon CPU E5-2650 v2

    Intel Xeon CPU E5-2650 v2 holds 8 Cores  x8 = 64  vCPUs.
    4 vCPU to each VM…64 vCPUs / 4 vCPU per VM = 16 VMs
    2 vCPU to each VM…64 vCPUs / 2 vCPU per VM = 32 VMs
    1 vCPUs to each VM…64 vCPUs / 1 vCPU per VM = 64 VMs
    

    here is the detailed explanation:

    That's a writeup based on some Microsoft article based on their Hyper-V.

    • I don't see a lot of Hyper-V based VPS here
    • Microsoft has probably no interest in downplaying their Hyper-V capabilities

    The secret sauce IMO lies more in old hardware pricing. That's why you see so many E5-26xx v2 based servers. Those (e.g. HP 1 HU gen 8) are dirt cheap (2nd or 3rd hand of course) servers offer a still decent performance and are an important ingredient for cheap VPS.

    Btw., the other secret sauce is the fact that most (I'd guess 90%) of customers very much over-estimate their real needs. Looking at the difference between what they think they need (e.g. at least 2 vCores and 1 Gb/s) and what they actually need (often like 50% of a vCore and not even 100Mb/s) creates a lot of opportunities.

  • nCPU=count_cpu("CPU", "vCPU");
    echo nCPU;

    1

    Thanked by 1ITLabs
  • ViridWebViridWeb Member, Host Rep

    @rchurch said:
    This vCPU issue has come up again for me.

    How much compute power is there in a vCPU? I remember when Amazon stated in the early days of EC2 that a VM was equivalent to 1.7Ghz AMD Opteron or some other such chip.

    There are all kinds of offers mentioning vCPUs here and there without stating exactly how much power is being offered.

    Some people say a vCPU is equivalent to a thread or whatever?

    Is the industry ever going to come out and standardize on what a vCPU is, relative to some known quantity, such as a Pentium D core clocked at 3.0Ghz?

    I just noticed the thread about Oracle's Free Tier which mentions an OCPU, which I assume if Oracle's idiosyncratic measure of a vCPU.

    Compute Classic - FAQ | Oracle Cloud Infrastructure

    An OCPU is defined as the CPU capacity equivalent of one physical core of an Intel Xeon processor with hyper threading enabled. For Intel Xeon processor each OCPU corresponds to two hardware execution threads, known as vCPUs.

    Is this the official definition of a vCPU?

    When providers on LET talk about cores, do the mean one of these vCPUs, which is also being shared with other users?

    It depends on your provider how he balances the resources..

  • @AlwaysSkint said:
    How long is a piece of string?

    you just remineded me of my English teacher... back in 1988-1989

    Thanked by 1AlwaysSkint
  • MGarbisMGarbis Member
    edited September 2019

    You have to ask it from your provider. There's no standard for that so it can be anything.

  • From a technical stand point, a vCPU is the equivalent of a thread or core of the underlaying CPU. But for all practical purposes, it is whatever the provider want it to be. It is the little dirty secret of how they can offer a 4 core VPS for as little as $5/month. I'd rather get a honest dedicated core (or fractional dedicated - say 30%) than a multi-core fair use.

    Benchmark the VPS when you first got it, and keep a keen eye on your CPU steal time.

  • @jbuggie said:
    ..
    Benchmark the VPS when you first got it, and keep a keen eye on your CPU steal time.

    Speaking of which, anyone else notice high steal on their OVH-CA VPS (cluster-001)?

  • @AlwaysSkint said:
    How long is a piece of string?

    Half the length times two..

  • @dahartigan said:
    Half the length times two..

    Crap, you read the same textbook.

    Thanked by 1dahartigan
  • uptimeuptime Member
    edited September 2019

    @raindog308 said:
    Dòngshān said, “Three pounds of flax.”

    sounds pretty mysterious, like some kind of transcendental absurdity, right?

    But I've heard it explained that the three pounds of flax refers to the fabric to make a monk's robe.

    So that's the meta-koan right there - it may be just a fairly pedestrian allusion to the concept of an empty suit (or ... whatever else a monk's robe might signify, I guess). But my western hipster intuitive interpretation of course has always been to parse it as some kind of Discordian-style surrealism. :blush:

    Anyhoo - about this vcore vs real hardware CPU core thing ...

    For most (though I suppose not all) applications the CPU isn't going to be running at full capacity all the time - it's waiting on disk i/o or network, etc, etc. So it's nice to be able to have some CPU burst capacity when needed and allow others to enjoy the same when the CPU would otherwise be idle - and benefiting by sharing the cost of the hardware. This is how a well-managed VPS offering "shared vcore" rather than dedicated CPU should work. It can get messy when there is excessive over-selling (though I'm guessing disk i/o may become the most obvious rate-limiting factor in many overly over-sold setups).

    One more angle to consider though - I wonder if anyone (perhaps @jsg ?) might be able to share some insight into the significance of cache memory in light of presumably increased context switching etc in the shared vcore scenario.

  • raindog308raindog308 Administrator, Veteran

    I’m still working on Joshu’s Dog.

  • @uptime said:

    @raindog308 said:
    Dòngshān said, “Three pounds of flax.”

    sounds pretty mysterious, like some kind of transcendental absurdity, right?

    But I've heard it explained that the three pounds of flax refers to the fabric to make a monk's robe.

    So that's the meta-koan right there - it may be just a fairly pedestrian allusion to the concept of an empty suit (or ... whatever else a monk's robe might signify, I guess). But my western hipster intuitive interpretation of course has always been to parse it as some kind of Discordian-style surrealism. :blush:

    Anyhoo - about this vcore vs real hardware CPU core thing ...

    For most (though I suppose not all) applications the CPU isn't going to be running at full capacity all the time - it's waiting on disk i/o or network, etc, etc. So it's nice to be able to have some CPU burst capacity when needed and allow others to enjoy the same when the CPU would otherwise be idle - and benefiting by sharing the cost of the hardware. This is how a well-managed VPS offering "shared vcore" rather than dedicated CPU should work. It can get messy when there is excessive over-selling (though I'm guessing disk i/o may become the most obvious rate-limiting factor in many overly over-sold setups).

    One more angle to consider though - I wonder if anyone (perhaps @jsg ?) might be able to share some insight into the significance of cache memory in light of presumably increased context switching etc in the shared vcore scenario.

    In various testings have found that I/O is often limiting factor in a good/dedicated CPU+ram vps. How does providers mitigate this? Will limiting iops help?

  • jsgjsg Member, Resident Benchmarker

    @uptime said:
    One more angle to consider though - I wonder if anyone (perhaps @jsg ?) might be able to share some insight into the significance of cache memory in light of presumably increased context switching etc in the shared vcore scenario.

    Well, it get's trashed, of course but depending on what one does that's usually not tragic. L1 caches are very "lively" things anyway but they are also fast. Where it gets relatively ugly is L3 which is much slower than L1 and L2 and much larger, too.

    Btw, what's much much worse is when a provider (I guess a few do it) pumps up memory with whatever type of storage (vmem, swap, however you call it).

    That's (part of why) my advice is to not care too much about the CPU (as in "the best and fastest only!!") but to look for a balanced system. So I don't care about your L3 being massive but I do care about your RAM being fast (relative to its type) and I want to see sensible values for L1 and L2. Also: more memory is almost always worth more (in terms of performance) than faster clock speed. That said I'd stay away from too low clockspeed like 1.7 or 1.8 Ghz. I guess the sweet spot is in the 2.3 - 2.7 GHz range.

    Real world and recent example: A (deservedly well respected) provider here had put (a) different speed, and (b) relatively slow RAM into a box with processor A while another box (at another provider) had processor B but with consistent and fast RAM. Result: processor B which is some generations older than proc. A delivered significantly better results.

  • How long is this thread going to get?

  • V=5 ∴ VCPU=7(42)/WSS-SYSTEMD

  • measure my stick.

  • uptimeuptime Member
    edited September 2019

    @jsg - thanks for that insight with regard to preferring a well-balanced system (with enough fast ram) rather than raw clock speed for CPU.

    @rchurch - I suppose another point to be made when comparing "vcore" to hardware core when dealing with different VPS offerings is: what flags are passed along in the emulation setup?

    One of the first things I do when evaluating a VPS is to check for the AES flag (grep aes /proc/cpuinfo) since that could have a significant effect on disk performance when using full-disk encryption (in addition to several other benefits). If the AES flag is not enabled in the default installation, then that's one of the few things I'll put a support ticket in to explain why I want it and see if it can be fixed for me. I have yet to find a provider unwilling to accommodate.

    Thanked by 1bikegremlin
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