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Remove --with-lg-tiny-min.

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This option isn't useful in practice.

This partially resolves #580.
This commit is contained in:
Jason Evans
2017-04-21 15:35:54 -07:00
parent b54530020f
commit af76f0e5d2
4 changed files with 11 additions and 41 deletions
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37
INSTALL
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@@ -233,38 +233,17 @@ any of the following arguments (not a definitive list) to 'configure':
This setting is architecture-specific, and although jemalloc includes known
safe values for the most commonly used modern architectures, there is a
wrinkle related to GNU libc (glibc) that may impact your choice of
<lg-quantum>. On most modern architectures, this mandates 16-byte alignment
(<lg-quantum>=4), but the glibc developers chose not to meet this
<lg-quantum>. On most modern architectures, this mandates 16-byte
alignment (<lg-quantum>=4), but the glibc developers chose not to meet this
requirement for performance reasons. An old discussion can be found at
https://sourceware.org/bugzilla/show_bug.cgi?id=206 . Unlike glibc,
jemalloc does follow the C standard by default (caveat: jemalloc
technically cheats if --with-lg-tiny-min is smaller than
--with-lg-quantum), but the fact that Linux systems already work around
this allocator noncompliance means that it is generally safe in practice to
let jemalloc's minimum alignment follow glibc's lead. If you specify
--with-lg-quantum=3 during configuration, jemalloc will provide additional
size classes that are not 16-byte-aligned (24, 40, and 56).
--with-lg-tiny-min=<lg-tiny-min>
Specify the base 2 log of the minimum tiny size class to support. Tiny
size classes are powers of 2 less than the quantum, and are only
incorporated if <lg-tiny-min> is less than <lg-quantum> (see
--with-lg-quantum). Tiny size classes technically violate the C standard
requirement for minimum alignment, and crashes could conceivably result if
the compiler were to generate instructions that made alignment assumptions,
both because illegal instruction traps could result, and because accesses
could straddle page boundaries and cause segmentation faults due to
accessing unmapped addresses.
The default of <lg-tiny-min>=3 works well in practice even on architectures
that technically require 16-byte alignment, probably for the same reason
--with-lg-quantum=3 works. Smaller tiny size classes can, and will, cause
crashes (see https://bugzilla.mozilla.org/show_bug.cgi?id=691003 for an
example).
This option is rarely useful, and is mainly provided as documentation of a
subtle implementation detail. If you do use this option, specify a
value in [3, ..., <lg-quantum>].
technically cheats for size classes smaller than the quantum), but the fact
that Linux systems already work around this allocator noncompliance means
that it is generally safe in practice to let jemalloc's minimum alignment
follow glibc's lead. If you specify --with-lg-quantum=3 during
configuration, jemalloc will provide additional size classes that are not
16-byte-aligned (24, 40, and 56).
The following environment variables (not a definitive list) impact configure's
behavior: