4 ms·

why does it break userland? if you need to know the page size, you should query sysconf SC_PAGESIZE.

akdev1l 2y ago
Assumptions in the software. Jemalloc is infamous for this: https://github.com/sigp/lighthouse/issues/5244 https://github.com/sigp/lighthouse/issues/5244
Dwedit 2y ago
Emulating a processor with 4K size pages becomes much higher performance if you can use real addresses directly.
fweimer 2y ago
It should not break userland. GNU/Linux (not necessarily Android though) has supported 64K pages pretty much from the start because that was the originally page size chosen for server-focus kernels and distributions. But there are some things that need to be worked around. Certain build processes determine the page size at compile time and assume it's the same at run time, and fail if it is not: https://github.com/jemalloc/jemalloc/issues/467 https://github.com/jemalloc/jemalloc/issues/467 Some memory-mapped files formats have assumptions about page granularity: https://bugzilla.redhat.com/show_bug.cgi?id=1979804 https://bugzilla.redhat.com/show_bug.cgi?id=1979804 The file format issue applies to ELF as well. Some people patch their toolchains (or use suitable linker options) to produce slightly smaller binaries that can only be loaded if the page size is 4K, even though the ABI is pretty clear in that you should link for compatibility with up to 64K pages.
ndesaulniers 2y ago
Ossification. If the page size has been 4k for decades for most OS' and architectures, people get sloppy and hard code that literal value, rather than query for it.
PhilipRoman 2y ago
Replacing compile time constants with function calls will always bring some trouble, suddenly you need to rearrange your structures, optimizations get missed (in extreme cases you can accidentally introduce a DIV instruction), etc. So it is not surprising that code assumes 4k pages.
- mgaunard 2y ago
  Any code that does divisions and modulos with non-constants that are known to be powers of 2 should do the optimization manually. Even for non-powers-of-two there are also techniques to speed up divisions if the same divisor is used repeatedly.

Assumptions in the software. Jemalloc is infamous for this: https://github.com/sigp/lighthouse/issues/5244 https://github.com/sigp/lighthouse/issues/5244

Dwedit 2y ago

Emulating a processor with 4K size pages becomes much higher performance if you can use real addresses directly.

fweimer 2y ago

It should not break userland. GNU/Linux (not necessarily Android though) has supported 64K pages pretty much from the start because that was the originally page size chosen for server-focus kernels and distributions. But there are some things that need to be worked around. Certain build processes determine the page size at compile time and assume it's the same at run time, and fail if it is not: https://github.com/jemalloc/jemalloc/issues/467 https://github.com/jemalloc/jemalloc/issues/467 Some memory-mapped files formats have assumptions about page granularity: https://bugzilla.redhat.com/show_bug.cgi?id=1979804 https://bugzilla.redhat.com/show_bug.cgi?id=1979804 The file format issue applies to ELF as well. Some people patch their toolchains (or use suitable linker options) to produce slightly smaller binaries that can only be loaded if the page size is 4K, even though the ABI is pretty clear in that you should link for compatibility with up to 64K pages.

ndesaulniers 2y ago

Ossification. If the page size has been 4k for decades for most OS' and architectures, people get sloppy and hard code that literal value, rather than query for it.

PhilipRoman 2y ago

Replacing compile time constants with function calls will always bring some trouble, suddenly you need to rearrange your structures, optimizations get missed (in extreme cases you can accidentally introduce a DIV instruction), etc. So it is not surprising that code assumes 4k pages.

mgaunard 2y ago
Any code that does divisions and modulos with non-constants that are known to be powers of 2 should do the optimization manually. Even for non-powers-of-two there are also techniques to speed up divisions if the same divisor is used repeatedly.