4 ms·

New RISC-V Svnapot Extension

24 points by hasheddan 5y ago

artemonster 5y ago
Can someone with RISC-V and CPU background do an ELI5 and why this is interesting/important?
- kragen 5y ago
  I don't understand it but it looks like an extension for 64k hugepages?
  - monocasa 5y ago
    It's a a generic solution for page size to be within a range of aligned power of two sizes, but only the bit pattern specifying 64k isn't reserved at the moment. It'll allow for all sorts of page sizes with fairly minimal TLB overhead when it's all said and done, rather than normal huge pages that tend to only increase in page table radix size (eg. on x86_64, 512x each level, ie. 4k->2MB->1GB granularity). It's real handy for reducing TLB overhead in the general case for the same reasons as why newer archs have larger pages, but in a way that shouldn't require a given os instance to have one base page table size so you don't need to recompile/port older programs that want 4k pages.
    - pm215 5y ago
      The twitter thread says it specifically doesn't change the size of a page, though. AIUI the idea is that it allows one TLB entry to cover more than just one page -- the page table entry promises "all 64K have the same permissions", so the TLB doesn't need to track every page in that 64K separately. That's useful because TLB entries are a limited resource and using more of them means we get more TLB misses and worse peformance. You could also economise on TLB use by making the page size larger, but then it has to be larger for the whole process, which has binary-compatibility implications and also can waste memory (eg with a 64K page size, even if you only need 4K to mmap a small file you have to use 64K anyway). Compare the Arm page table 'contiguous hint bit', which achieves the same thing in a slightly different way.
      - deleted 5y ago
        [deleted]
      - monocasa 5y ago
        Technically true, but code running on those mappings won't be able to tell the difference. Another way to look at it is that the page size does change, but the size of the page table entry expands at the same rate, and you can therefore mix and match page sizes. They're equivalent concepts.
metadat 5y ago
Man these twitter "Blog Posts" are tough to read, especially on mobile devices. After getting through the Install-Our-App nag: There is so much visual noise, reading the article one or two sentences at a time and scrolling constantly is a totally disjointed experience for me. Sucks because I am in fact interested in this topic and would like to broaden my understanding! Sorry for the rant. Happier New Year to you all! -- Twitter's incessant modal dialogue app-install nag is a form of punishment / user-abuse, because the company should know by now, based on the dozens of previous rejections: I'm not interested! Too much trouble to set a goddamn cookie? Asshole product managers and their prescious "metrics" are running (ruining) the show.
- LargoLasskhyfv 5y ago
  Why don't you try exchanging any twitter.com with nitter.net while leaving the rest as it is? Like this: https://nitter.net/hasheddan/status/1476950148630601735 https://nitter.net/hasheddan/status/1476950148630601735

24 points by hasheddan 5y ago

artemonster 5y ago

Can someone with RISC-V and CPU background do an ELI5 and why this is interesting/important?

kragen 5y ago
I don't understand it but it looks like an extension for 64k hugepages?
- monocasa 5y ago
  It's a a generic solution for page size to be within a range of aligned power of two sizes, but only the bit pattern specifying 64k isn't reserved at the moment. It'll allow for all sorts of page sizes with fairly minimal TLB overhead when it's all said and done, rather than normal huge pages that tend to only increase in page table radix size (eg. on x86_64, 512x each level, ie. 4k->2MB->1GB granularity). It's real handy for reducing TLB overhead in the general case for the same reasons as why newer archs have larger pages, but in a way that shouldn't require a given os instance to have one base page table size so you don't need to recompile/port older programs that want 4k pages.
  - pm215 5y ago
    The twitter thread says it specifically doesn't change the size of a page, though. AIUI the idea is that it allows one TLB entry to cover more than just one page -- the page table entry promises "all 64K have the same permissions", so the TLB doesn't need to track every page in that 64K separately. That's useful because TLB entries are a limited resource and using more of them means we get more TLB misses and worse peformance. You could also economise on TLB use by making the page size larger, but then it has to be larger for the whole process, which has binary-compatibility implications and also can waste memory (eg with a 64K page size, even if you only need 4K to mmap a small file you have to use 64K anyway). Compare the Arm page table 'contiguous hint bit', which achieves the same thing in a slightly different way.
    - deleted 5y ago
      [deleted]
    - monocasa 5y ago
      Technically true, but code running on those mappings won't be able to tell the difference. Another way to look at it is that the page size does change, but the size of the page table entry expands at the same rate, and you can therefore mix and match page sizes. They're equivalent concepts.

metadat 5y ago

Man these twitter "Blog Posts" are tough to read, especially on mobile devices. After getting through the Install-Our-App nag: There is so much visual noise, reading the article one or two sentences at a time and scrolling constantly is a totally disjointed experience for me. Sucks because I am in fact interested in this topic and would like to broaden my understanding! Sorry for the rant. Happier New Year to you all! -- Twitter's incessant modal dialogue app-install nag is a form of punishment / user-abuse, because the company should know by now, based on the dozens of previous rejections: I'm not interested! Too much trouble to set a goddamn cookie? Asshole product managers and their prescious "metrics" are running (ruining) the show.

LargoLasskhyfv 5y ago
Why don't you try exchanging any twitter.com with nitter.net while leaving the rest as it is? Like this: https://nitter.net/hasheddan/status/1476950148630601735 https://nitter.net/hasheddan/status/1476950148630601735