4 ms·

Not sure if that's true any longer on recent kernels. There have been some changes with folios that I think enable huge pages in the page cache. $ cat /proc/

Not sure if that's true any longer on recent kernels. There have been some changes with folios that I think enable huge pages in the page cache. $ cat /proc/<firefox process>/smaps [...] 7efca62e3000-7efcaa13d000 r-xp 00ae3000 00:18 75354786 /usr/lib/libLLVM-15.so Size: 63848 kB KernelPageSize: 4 kB MMUPageSize: 4 kB Rss: 58420 kB Pss: 19213 kB Pss_Dirty: 0 kB Shared_Clean: 56372 kB Shared_Dirty: 0 kB Private_Clean: 2048 kB Private_Dirty: 0 kB Referenced: 58420 kB Anonymous: 0 kB LazyFree: 0 kB AnonHugePages: 0 kB ShmemPmdMapped: 0 kB FilePmdMapped: 57344 kB <==== huge pages Shared_Hugetlb: 0 kB Private_Hugetlb: 0 kB Swap: 0 kB SwapPss: 0 kB Locked: 0 kB THPeligible: 1 VmFlags: rd ex mr mw me sd

scottlamb 4y ago
Exciting if true! I see https://docs.kernel.org/filesystems/proc.html https://docs.kernel.org/filesystems/proc.html describes FilePmdMapped as "Page cache mapped into userspace with huge pages", consistent with what you are saying. I don't fully understand the distinction between that and FileHugePages: "Memory used for filesystem data (page cache) allocated with huge pages". I wouldn't think it'd be possible to map it into userspace as huge pages if the kernel hasn't allocated it as contiguous physical memory (and consistently aligned with the userspace virtual addresses), so there's something I'm missing. What kernel version did that output come from? Do you happen to know if Firefox did anything special to set that up? What filesystem type is this?
- scottlamb 4y ago
  Huh. I did a little digging through kernel source. There's been a CONFIG_READ_ONLY_THP_FOR_FS since 2019. It's still marked as experimental and isn't enabled on the precompiled kernel I'm using (with Ubuntu 22.10). Is that option set on your kernel, or is this something else? I also see something about MADV_COLLAPSE that supposedly supports file-backed pages. [1] [1] https://lwn.net/Articles/913363/ https://lwn.net/Articles/913363/
  - the8472 4y ago
    Yeah, it's enabled here $ zgrep 'CONFIG_READ_ONLY_THP_FOR_FS' /proc/config.gz CONFIG_READ_ONLY_THP_FOR_FS=y > What kernel version did that output come from? kernel 6.2.8-arch1-1 > What filesystem type is this? btrfs > I don't fully understand the distinction between that and FileHugePages: "Memory used for filesystem data (page cache) allocated with huge pages". Probably pages in the page cache that aren't mapped into a user process. Which is what happens when you read()

scottlamb 4y ago

Exciting if true! I see https://docs.kernel.org/filesystems/proc.html https://docs.kernel.org/filesystems/proc.html describes FilePmdMapped as "Page cache mapped into userspace with huge pages", consistent with what you are saying. I don't fully understand the distinction between that and FileHugePages: "Memory used for filesystem data (page cache) allocated with huge pages". I wouldn't think it'd be possible to map it into userspace as huge pages if the kernel hasn't allocated it as contiguous physical memory (and consistently aligned with the userspace virtual addresses), so there's something I'm missing. What kernel version did that output come from? Do you happen to know if Firefox did anything special to set that up? What filesystem type is this?

scottlamb 4y ago
Huh. I did a little digging through kernel source. There's been a CONFIG_READ_ONLY_THP_FOR_FS since 2019. It's still marked as experimental and isn't enabled on the precompiled kernel I'm using (with Ubuntu 22.10). Is that option set on your kernel, or is this something else? I also see something about MADV_COLLAPSE that supposedly supports file-backed pages. [1] [1] https://lwn.net/Articles/913363/ https://lwn.net/Articles/913363/
- the8472 4y ago
  Yeah, it's enabled here $ zgrep 'CONFIG_READ_ONLY_THP_FOR_FS' /proc/config.gz CONFIG_READ_ONLY_THP_FOR_FS=y > What kernel version did that output come from? kernel 6.2.8-arch1-1 > What filesystem type is this? btrfs > I don't fully understand the distinction between that and FileHugePages: "Memory used for filesystem data (page cache) allocated with huge pages". Probably pages in the page cache that aren't mapped into a user process. Which is what happens when you read()