Investigating Performance of Multi-Threading on Zen 3 and AMD Ryzen 5000

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• CityBlue - Thursday, December 3, 2020 - link

  Sure, there are numerous examples of non-core silicon in x86 environments that are insecure, however this article is about SMT yet it avoids mentioning the ever growing list of security problems with Intels implementation of SMT. The Intel implementation of SMT is so badly flawed from a security perspective that the only way to secure Intel CPUs is to completely disable SMT, and that's the bottom line recommendation of many kernel and distribution developers that have been trying to "fix" Intel CPUs for the past few years.

• abufrejoval - Thursday, December 3, 2020 - link

  I use an Intel i7 in my pfSense firewall appliance, which based on BSD.
  
  BSD tends to remind you, that you should run it with SMT disabled because of these side channel exposure security issues.
  
  Yet, with the only workload being pfSense and no workload under an attacker's control able to sniff data, I just don't see why SMT should be a risk there, while extra threads to deeply inspect with Suricata should help avoiding that deeper analysis creates a bottleneck in the uplink.
  
  You need to be aware of the architectural risks that exist on the CPUs you use, but to argue that SMT should always be off is a bit too strong.
  
  Admittedly, when you have 16 real cores to play with, disabling SMT hurts somewhat less than on an i3-7350K (2C/4T), a chip I purchased once out of curiosity, only to have it replaced by an i5-7600K (4C/4T) just before Kaby Lake left the shelves and became temptingly cheap.
  
  It held up pretty well, actually, especially because it did go to 4,8 GHz without more effort than giving it permission to turbo. But I'm also pretty sure the 4 real cores of the i5-7600k will let the system live longer as my daughter's gaming rig.

• CityBlue - Thursday, December 3, 2020 - link

  > to argue that SMT should always be off is a bit too strong.
  
  Not really - if you're a kernel or distro developer then Intel SMT "off" is the only sane recommendation you can give to end users given the state of Intel CPUs released over the last 10 years (note: the recommendation isn't relating to SMT in general, not even AMD SMT, it is only Intel SMT).
  
  However if end users with Intel CPUs choose to ignore the recommendation then that's their choice, as presumably they are doing so while fully informed of the risks.

• leexgx - Saturday, December 5, 2020 - link

  The SMT risk is more a server issue then a consumer issue

• schujj07 - Thursday, December 3, 2020 - link

  This article wasn't talking about Intel's implementation, only SMT performance on Zen 3. If this were about SMT on Intel then it would make sense, otherwise no.

• CityBlue - Thursday, December 3, 2020 - link

  The start of the article is discussing the pros and cons of SMT *in general* and then discusses where SMT is used, and where it is not used, giving examples of Intel x86 CPUs. Why not then mention the SMT security concerns with Intel CPUs too? That's a rhetorical question btw, we all know the reason.

• schujj07 - Friday, December 4, 2020 - link

  Since this is an AMD focused article, there isn't the side channel attack vector for SMT. Therefore why would you mention side channel attacks for Intel CPUs? That doesn't make any sense since Intel CPUs are only stated for who uses SMT and Intel's SMT marketing name. Hence bringing up Intel and side channel attack vectors would be including extraneous data/information to the article and take away from the stated goal. "In this review, we take a look at AMD’s latest Zen 3 architecture to observe the benefits of SMT."

• mode_13h - Sunday, June 6, 2021 - link

  > The Intel implementation of SMT is so badly flawed from a security perspective that
  > the only way to secure Intel CPUs is to completely disable SMT
  
  That's not true. The security problems with SMT are all related to threads from different processes sharing the same physical core. To this end, the Linux kernel now has an option not to do that, since it's the kernel which decides what threads run where. So, you can still get most of the benefits of SMT, in multithreaded apps, but without the security risks!

• dotjaz - Thursday, December 3, 2020 - link

  Windows knows how to allocate threads to the same CCX (after patches of course). It not only knows the physical core, it also knows the topology.

• leexgx - Saturday, December 5, 2020 - link

  A lot of people forget to install the amd chipset drivers witch can result in some small loss of performance (but also need bios to be kept upto date as well Co compleat the ccx group support and best cores support to advertise to windows