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cmuratori's profile
Casey Muratori
Casey Muratori
Casey Muratori
@cmuratori

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Casey Muratori

@cmuratori

I'm worried that the baby thinks people can't change.

Seattle
caseymuratori.com
Joined March 2009

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    1. Casey Muratori‏ @cmuratori May 11

      In case there are any NT kernel devs listening: from cold start, an exe that touches ~1gb of memory takes over 100ms to do so due to page handling. With 2MB pages enabled, this drops down to 30ms. This suggests to me that a "MEM_REALLY_COMMIT" VirtualAlloc flag would help?

      3 replies 0 retweets 29 likes
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    2. Casey Muratori‏ @cmuratori May 11

      Because when you VirtualAlloc 1gb, if we know the total time plus page faulting to use the memory is 30ms with 2MB pages, one assumes that even with 4k pages if VirtualAlloc did the page prep right there in bulk, it could at least provide something closer to 30ms?

      1 reply 0 retweets 10 likes
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    3. Stefan Reinalter‏ @molecularmusing May 11
      Replying to @cmuratori

      Don't you think it's simply the price you have to pay for setting up 512 times as many page table entries? And could PrefetchVirtualMemory help in your case (don't know if you're doing I/O or just reserving pages)?

      3 replies 0 retweets 0 likes
    4. Casey Muratori‏ @cmuratori May 11
      Replying to @molecularmusing

      I will try adding a PVM right after the VirtualAlloc and see if anything changes. As for it taking 80ms to set up the pagetable entries, while _possible_, we are talking about 300 million cycles here. Maybe that is the cost of 1gb of 4k pagetable entries, but I doubt it?

      1 reply 0 retweets 1 like
    5. Casey Muratori‏ @cmuratori May 11
      Replying to @cmuratori @molecularmusing

      So, absent a kernel dev telling me otherwise, my assumption would definitely be that most of the time is in repeatedly handling faults and doing the pages serially, instead of once in bulk.

      1 reply 0 retweets 1 like
    6. Krzysztof Lesiak‏ @CarePackage17 May 11
      Replying to @cmuratori @molecularmusing

      How do you measure? If you make an ETW trace and open in WPA with symbols, you should be able to verify your assumption pretty quickly. (There'd be some ntoskrnl!KiPageFault frames in your stack I think)

      1 reply 0 retweets 0 likes
    7. Stefan Reinalter‏ @molecularmusing May 11
      Replying to @CarePackage17 @cmuratori

      Second that. What I do find weird is that I would have thought that most of the time is spent on zeroing the pages, but then you wouldn't see such a difference between 4k and 2MB pages - in both cases, 1GB has to be zeroed.

      2 replies 0 retweets 0 likes
    8. Casey Muratori‏ @cmuratori May 11
      Replying to @molecularmusing @CarePackage17

      I don't think NT kernel spends any time zeroing pages from a cold start, unless you have been doing a lot with the machine recently. It background-clears pages so they are ready - it doesn't wait until you ask for them.

      1 reply 0 retweets 1 like
    9. Stefan Reinalter‏ @molecularmusing May 11
      Replying to @cmuratori @CarePackage17

      Ah yes, you're right, I remember reading about this on Bruce Dawson's blog (which I guess you know already):https://randomascii.wordpress.com/2014/12/10/hidden-costs-of-memory-allocation/#more-1814 …

      1 reply 0 retweets 4 likes
      Casey Muratori‏ @cmuratori May 11
      Replying to @molecularmusing @CarePackage17

      Pretty sure it is well documented by the NT devs - or at least, I feel like I remember reading some official breakdowns of how the page provisioning works.

      12:28 PM - 11 May 2021
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Stefan Reinalter‏ @molecularmusing May 11
          Replying to @cmuratori @CarePackage17

          In any of the Windows Internals books perhaps? Unfortunately I can't check right now, because I'm not in the office.

          1 reply 0 retweets 0 likes
        3. Peter Lindgren‏ @burktomat May 11
          Replying to @molecularmusing @cmuratori @CarePackage17

          https://channel9.msdn.com/Events/TechEd/NorthAmerica/2011/WCL405 … This talk by @markrussinovich is great and part 2 goes into detail on page zeroing (it's via bg proc..). It's from win7 though iirc so it might be outdated now.

          0 replies 0 retweets 0 likes
        4. End of conversation

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