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Add --with-lg-tiny-min, generalize --with-lg-quantum.

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Jason Evans
2014-10-10 22:34:25 -07:00
parent 2eb941a3d3
commit 81e547566e
6 changed files with 105 additions and 16 deletions
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32
INSTALL
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@@ -230,10 +230,9 @@ any of the following arguments (not a definitive list) to 'configure':
roughly <lg-size-class-group>=4, depending on page size.
--with-lg-quantum=<lg-quantum>
Specify the base 2 log of the minimum allocation alignment (only
<lg-quantum>=3 and <lg-quantum>=4 are supported). jemalloc needs to know
the minimum alignment that meets the following C standard requirement
(quoted from the April 12, 2011 draft of the C11 standard):
Specify the base 2 log of the minimum allocation alignment. jemalloc needs
to know the minimum alignment that meets the following C standard
requirement (quoted from the April 12, 2011 draft of the C11 standard):
The pointer returned if the allocation succeeds is suitably aligned so
that it may be assigned to a pointer to any type of object with a
@@ -247,8 +246,8 @@ any of the following arguments (not a definitive list) to 'configure':
(<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 by only providing 8-byte alignment for 8-byte allocation requests),
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
@@ -256,6 +255,27 @@ any of the following arguments (not a definitive list) to 'configure':
are not 16-byte-aligned (24, 40, and 56, assuming
--with-lg-size-class-group=2).
--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>].
The following environment variables (not a definitive list) impact configure's
behavior: