The "5-level Page Table" Default in Linux May Impact Throughput

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Mark E. Dawson, Jr.

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Jun 17, 2022, 1:06:42 PM6/17/22
to mechanical-sympathy
In the article below, I address just *one* of the areas where this new default Linux build option in most recent distros can adversely impact multithreaded performance - page fault handling. But any workload which requires the kernel to mimic MMU page table walking to accomplish a task could be impacted adversely, as well (e.g., pipe communication). You'd do well to do your own testing:


NOTE: 5-level Page Table can be disabled with "no5lvl" on the kernel boot command line.

Gil Tene

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Jun 18, 2022, 11:30:10 AM6/18/22
to mechanical-sympathy
A good read, and very well written. 

I'm with you in general on most/all of it except for one thing: That 64TB line. 

I draw that same line at 256GB. Or at least at a 256GB guest/pod/container/process. There are things out there that use virtual memory multi (and "many") mapping tricks for speed, and eat up a bunch of that seemingly plentiful 47 bit virtual user space in the process. The ones I know the most about (because I am mostly to blame for them) are high throughput concurrent GC mechanisms. Those have a multitude of implementation variants, all of which encode phases/generations/spaces/colors in higher-order virtual bits and use multi-mapping to efficiently recycle physical memory.

For a concrete example, when running on a vanilla linux kernel with 4-level page tables, the current C4 collector in the Prime JVM (formerly known as "Zing"), uses different phase encoding, and different LVB barrier instruction encodings, depending on whether the heap size is above or below 256GB. Below 256GB, C4 gets to use sparse phase and generation encodings (using up 6 bits of virtual space) and a faster LVB test (test&jmp), and above 256GB, it uses denser encodings (using up only 3 bits) with slightly more expensive LVBs (test a bit in a mask & jmp). A 5 level page table (on hardware that supports it) we can move that line out by 512x, which means that even many-TB Java heaps can use the same cheap LVB tests that the smaller ones do.

I expect the (exact) same cosniderations will be true for ZGC in OpenJDK (once it adds a generational mode to be able to keep up with high throughout allocations and large live sets), as ZGC's virtual space encoding needs and resulting LVB test instruction encodings are identical to C4's.

So I'd say you can safely turn off 5 level tables on machines that physical have less than 256GB of memory, or on machines that are known to not run Java (now or in the future), or some other in-memory application technology that uses virtual memory tricks at scale. But above 256GB, I'd keep it on, especially if the thing is e.g. a Kubernetes node that may want to run some cool Java workload tomorrow with the best speed and efficiency.

Mark Dawson

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Jun 18, 2022, 2:58:00 PM6/18/22
to mechanica...@googlegroups.com
Thanks, Gil, for the compliment!

Regarding the VM tricks used by certain GCs out there, wouldn't that be better served instead by Intel's upcoming Linear Address Masking (LAM) in Sapphire Rapids, which would allow you to apply all types of tricks to the upper 15 bits of unused bits on 4-level Page Table systems (only the upper 7 bits on 5-level Page Table systems)? With LAM, user applications no longer need to worry about masking off updates to those bits when accessing memory via those pointers since the system will ignore them (i.e., the CPU no longer checks for canonicality).

In that case, 4-level Page Tables maintain their advantage over 5-level Page Tables provided an end user does not require the 64TB or more of RAM which 5-level Page Tables enable.


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Gil Tene

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Jun 19, 2022, 11:45:06 AM6/19/22
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Yes yes yes. I want LAM. I've wanted it (and have been asking for it or something like it on x86) for over 14 years. And *when* LAM becomes a real thing we can run on and manipulate from user mode, I'll absolutely prefer using it to what we do today with VM multi-mapping, for many many reasons. Including (but not limited to) not having to deal with vma count limits, virtual memory fragmentation (and "innovative" ways to cap it and defragment it), cost of creating/changing mappings, etc. etc., and not having to explain to people that Linux's RSS is one of those words they use a lot, and that I do not think it means what they think it means (but that e.g. cgroup memory use is a thing tracks what they think RSS means but doesn't), etc. etc. 

And when LAM is available, I''d much prefer 4-level page table to 5-level, because it would give me 9 more LAM-provided metadata bits to play with...

But until LAM becomes a thing (in hardware lots of people run on, with Linux kernel and glibc support in Linux distros people actually run their OS images and container hosts on, etc.), 5-level page tables currently add speed (when dealing with >256GB of heap) on modern JVMs running on actual production x86 servers. And I think there may be other (non-Java) workloads for which the same is true. So I personally like 5 level page tables being on by default (on hardware that supports them) and prefer that people not turn them off unless their OS images are provisioned with physical memory small enough that they have no value even there. I.e. for physical machines or VM images with <256GB or RAM, turning off level 5 page tables makes perfect sense. But above that, I like them on better than I like them off.

BTW, several other processors have (and had) this masking or "don't careness" stuff that LAM will finally be bringing to x86, and on those processors, we have (and had) better options than doing more than 2-way VM multi-mapping. But since the vast majority of the world runs on x86, we do what we must to makes things work there, and that makes us do some funny stuff with virtual memory, and some a-little-slower funnier stuff (with 4 level page tables) when heap sizes go above sizes that, even when not super-common, are very real today. 5-level page tables move the line where we need the  a-little-slower funnier stuff out ot well beyond the RAM sizes mortals can get their hands on today. Hopefully LAM arrives before real-world RAM grows too much even for that.

Mark Dawson

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Jun 19, 2022, 12:00:03 PM6/19/22
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Yes, I'm aware that ARM has offered LAM since long ago. And regarding LAM on Intel, we shouldn't have to wait that long at all since it's due in Sapphire Rapids.

As far as Java workloads go, it seems there are some runtimes that *require* disabling 5-level Page Tables already due to the fact that some runtimes already play pointer tagging tricks on the upper bits for metadata-tracking purposes and can't spare losing the extra 9 bits. Azul Zing even requires disabling it if using ZST (though I'm unsure why that is, specifically). All this, coupled with your feedback on some GC inner workings, makes me think the Java world certainly adds some complication to the discussion. Perhaps I'll write a followup article discussing these special case scenarios.

But the gist of my article is mainly about the generic performance impact, and things like page faults and pipe throughput and munmap() will certainly take a hit.

But thanks again for compliment on the content and the insightful feedback!

On Sun, Jun 19, 2022, 10:42 AM 'Gil Tene' via mechanical-sympathy <mechanica...@googlegroups.com> wrote:
Yes yes yes. I want LAM. I've wanted it (and have been asking for it or something like it on x86) for over 14 years. And *when* LAM becomes a real thing we can run on and manipulate form user mode, I'll absolutely prefer using it to what we do today with VM multi-mapping, for many many reasons. Including (but not limited to) not having to deal with vma count limits, virtual memory fragmentation (and "innovative" ways to cap it and defragment it), cost of creating/changing mappings, etc. etc., and not having to explain to people that Linux's RSS is one of those words they use a lot, and that I do not think it mean what they think it means (but that e.g. cgroup memory use is a thing tracks what they think RSS means but doesn't), etc. etc. 

And when LAM is available, I''d much prefer 4-level page table to 5-level, because it would give me 9 more LAM-provided metadata bits to play with...

But until LAM becomes a thing (in hardware lots of people run on, with Linux kernel and glibc support in Linux distros people actually run their OS images and container hosts on, etc.), 5-level page tables currently add speed (when dealing with >256GB of heap) on modern JVMs running on actual production x86 servers. And I think there may be other (non-Java) workloads for which the same is true. So I personally like 5 level page tables being on by default (on hardware that supports them) and prefer that people not turn them off unless their OS images are provisioned with physical memory small enough that they have no value even there. I.e. for physical machines or VM images with <256GB or RAM, turning off level 5 page tables makes perfect sense. But above that, I like them on better than I like them off.

BTW, several other processors have (and had) this masking or "don't careness" stuff that LAM will finally be bringing to x86, and on those processors, we have (and had) better options than doing more than 2-way VM multi-mapping. But since the vast majority of the world runs on x86, we do what we must to makes things work there, and that makes us do some funny stuff with virtual memory, and some a-little-slower funnier stuff (with 4 level page tables) when heap sizes go above sizes that, even when not super-common, are very real today. 5-level page tables move the line where we need the  a-little-slower funnier stuff out ot well beyond the RAM sizes mortals can get their hands on today. Hopefully LAM arrives before real-world RAM grows too much even for that.
 
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