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OpenBSD disables Intel's hyperthreading due to security concerns
61–70 of 158 posts
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#62Earlier quoted context omitted.
Building software, serving web pages, executing database queries, running a DOM layout, managing game logic... I mean, come on. You knew what I meant. Those are all tasks with "medium" cache residency and "occasional" stalls on DRAM. Anything that does a bunch of different things with a big-ish world of data. Conversely: finding a task that is L1-cache-bound but does not frequently have to stall for memory is much ha…
Oh, you meant typical for you. One task that is L1 cache bound and does not frequently stall for memory (if you code it up well) is matrix multiply.
> Oh, you meant typical for you.
I'm pretty sure those are meant to be, and I think are, "typical" for the general purpose CPU in use, and thus the general case.
Both mobile and desktop CPUs will be doing DOM layout, DB queries (whether to SQLite or the registry or just the filesystem), and possibly computing game logic on a regular basis.
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#63Earlier quoted context omitted.
Oh, you meant typical for you. One task that is L1 cache bound and does not frequently stall for memory (if you code it up well) is matrix multiply.
> Oh, you meant typical for you. I'm pretty sure those are meant to be, and I think are, "typical" for the general purpose CPU in use, and thus the general case. Both mobile and desktop CPUs will be doing DOM layout, DB queries (whether to SQLite or the registry or just the filesystem ), and possibly computing game logic on a regular basis.
It's becoming popular to want to push machine learning tasks onto edge devices like mobile and desktop CPUs, for example apps that include some machine learning. Some of these machine learning algorithms do a lot of matrix multiplies.
"Typical" is highly varied, and it changes.
Edit: here's an example: Google brings on-device machine learning to mobile with TensorFlow Lite
https://thenextweb.com/artificial-intelligence/2017/11/15/go...
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#64Earlier quoted context omitted.
> Oh, you meant typical for you. I'm pretty sure those are meant to be, and I think are, "typical" for the general purpose CPU in use, and thus the general case. Both mobile and desktop CPUs will be doing DOM layout, DB queries (whether to SQLite or the registry or just the filesystem ), and possibly computing game logic on a regular basis.
It's becoming popular to want to push machine learning tasks onto edge devices like mobile and desktop CPUs, for example apps that include some machine learning. Some of these machine learning algorithms do a lot of matrix multiplies. "Typical" is highly varied, and it changes. Edit: here's an example: Google brings on-device machine learning to mobile with TensorFlow Lite https://thenextweb.com/artificial-intelligen…
Would they be using mostly CPU for that, or would they offload it to the GPU or a dedicated chip? I would assume you would use your general purpose CPU only if all else wasn't available (and generally there's a GPU available on most end user devices these days).
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#65So... they "strongly suspect" (but don't know and haven't shown) there may be a Spectre-class bug enabled by current HT implementations and improving their scheduler is hard, so they'll pre-emptively disable HT outright on Intel CPUs now and others in the near future? I'm not an OpenBSD user (and glad for it, if this is anything to go by), but I'm curious - is this really how they operate, or does this decision stand…
Maybe they've got a heads up that there are more spectre like bugs incoming but not enough information to actually mitigate them.
CPU bugs seem to be a rich vein to mine at the moment.
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#66So... they "strongly suspect" (but don't know and haven't shown) there may be a Spectre-class bug enabled by current HT implementations and improving their scheduler is hard, so they'll pre-emptively disable HT outright on Intel CPUs now and others in the near future? I'm not an OpenBSD user (and glad for it, if this is anything to go by), but I'm curious - is this really how they operate, or does this decision stand…
> I'm not an OpenBSD user (and glad for it, if this is anything to go by),
.. yet most likely if interacting with unix systems rely on OpenSSH.
why would relying on a feature from a vendor with known processor security issues, including undisclosed hidden application processors, for a feature which has marginal performance improvement and in some cases degredation be a preferable stance?
at best ambivalence towards this decision would be the position to take, esp. given the very recent 'oh hey fpu registers are also a problem' "discovery" which they were entirely correct about..
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#67Earlier quoted context omitted.
>This suggests a long-term compromise solution where threads within a process can use hyperthreading to share a core, but threads in different processes can't. Given that hyperthreads share L1 cache, this might also be better for performance. Intuitively this may sound logical, however in practice it's often not the case. For many workloads putting two threads of the same program on a core ends up being worse than co…
In practice it sometimes is the case, though. SMT/hyperthreading is complicated. If you have a workload dominated by non-local DRAM fetches, it's a huge win because when the CPU pipeline is stalled on one thread it can still issue instructions from the other. If you have a workload dominated by L1 cache bandwidth, the opposite is true because the threads compete for the same resource. On balance, on typical workloads…
> workload dominated by non-local DRAM fetches,
How often is that a polite way of saying "software that is inefficient"?
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#68Earlier quoted context omitted.
It's becoming popular to want to push machine learning tasks onto edge devices like mobile and desktop CPUs, for example apps that include some machine learning. Some of these machine learning algorithms do a lot of matrix multiplies. "Typical" is highly varied, and it changes. Edit: here's an example: Google brings on-device machine learning to mobile with TensorFlow Lite https://thenextweb.com/artificial-intelligen…
Would they be using mostly CPU for that, or would they offload it to the GPU or a dedicated chip? I would assume you would use your general purpose CPU only if all else wasn't available (and generally there's a GPU available on most end user devices these days).
If possible the GPU, but not all GPUs have either a library or enough documentation to write one. I’ve seen complaints about this issue on mobile GPUs for years, no idea how widespread it is now.
BTW, this is just one example algorithm that I picked because it does (on the cpu) what the person I replied to said was rare.
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#69They should make it easier to find the diff behind all openbsd emails. I can’t find this one.
Re: OpenBSD disables Intel's hyperthreading due to security concerns
#70Earlier quoted context omitted.
And it's a mistake. Just recently I noticed that when Firefox loads multiple tabs of the same wordpress site, it starts hanging not unlike Firefox always used to hang. That's likely because it groups all of those same site pages under one process. I've never experienced that with Chrome. This is why I hope Firefox eventually (ASAP) switches to one process per tab, too. I can handle the browser using an extra GB of RA…
So, you're saying because you think you've discovered one case where there might be a problem, Firefox should completely change their architecture? And you're saying this in a discussion which frequently mentions how extremely varied workloads are?
No, man. He said Firefox should change their architecture, and he gave some kind of example.
After the way you seized on the word "typical", I kind of expected you to take words at face value. I didn't see any text to the effect that he thinks his say so is good enough.
Also, you're the one frequently mentioning how varied workloads are, and you don't constitute the discussion.
I'm going to go drink some cocoa to wash down this hook, line, and sinker I swallowed with your flame bait.