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xz: Limit --memlimit-compress to at most 4020 MiB for 32-bit xz.

See the code comment for reasoning. It's far from perfect but
hopefully good enough for certain cases while hopefully doing
nothing bad in other situations.

At presets -5 ... -9, 4020 MiB vs. 4096 MiB makes no difference
on how xz scales down the number of threads.

The limit has to be a few MiB below 4096 MiB because otherwise
things like "xz --lzma2=dict=500MiB" won't scale down the dict
size enough and xz cannot allocate enough memory. With
"ulimit -v $((4096 * 1024))" on x86-64, the limit in xz had
to be no more than 4085 MiB. Some safety margin is good though.

This is hack but it should be useful when running 32-bit xz on
a 64-bit kernel that gives full 4 GiB address space to xz.
Hopefully this is enough to solve this:

https://bugzilla.redhat.com/show_bug.cgi?id=1196786

FreeBSD has a patch that limits the result in tuklib_physmem()
to SIZE_MAX on 32-bit systems. While I think it's not the way
to do it, the results on --memlimit-compress have been good. This
commit should achieve practically identical results for compression
while leaving decompression and tuklib_physmem() and thus
lzma_physmem() unaffected.
This commit is contained in:
Lasse Collin 2020-02-01 19:56:18 +02:00
parent ba76d67585
commit 3539705108
2 changed files with 51 additions and 2 deletions

View file

@ -68,9 +68,39 @@ hardware_memlimit_set(uint64_t new_memlimit,
new_memlimit = (uint32_t)new_memlimit * total_ram / 100; new_memlimit = (uint32_t)new_memlimit * total_ram / 100;
} }
if (set_compress) if (set_compress) {
memlimit_compress = new_memlimit; memlimit_compress = new_memlimit;
#if SIZE_MAX == UINT32_MAX
// FIXME?
//
// When running a 32-bit xz on a system with a lot of RAM and
// using a percentage-based memory limit, the result can be
// bigger than the 32-bit address space. Limiting the limit
// below SIZE_MAX for compression (not decompression) makes
// xz lower the compression settings (or number of threads)
// to a level that *might* work. In practice it has worked
// when using a 64-bit kernel that gives full 4 GiB address
// space to 32-bit programs. In other situations this might
// still be too high, like 32-bit kernels that may give much
// less than 4 GiB to a single application.
//
// So this is an ugly hack but I will keep it here while
// it does more good than bad.
//
// Use a value less than SIZE_MAX so that there's some room
// for the xz program and so on. Don't use 4000 MiB because
// it could look like someone mixed up base-2 and base-10.
const uint64_t limit_max = UINT64_C(4020) << 20;
// UINT64_MAX is a special case for the string "max" so
// that has to be handled specially.
if (memlimit_compress != UINT64_MAX
&& memlimit_compress > limit_max)
memlimit_compress = limit_max;
#endif
}
if (set_decompress) if (set_decompress)
memlimit_decompress = new_memlimit; memlimit_decompress = new_memlimit;

View file

@ -5,7 +5,7 @@
.\" This file has been put into the public domain. .\" This file has been put into the public domain.
.\" You can do whatever you want with this file. .\" You can do whatever you want with this file.
.\" .\"
.TH XZ 1 "2019-05-11" "Tukaani" "XZ Utils" .TH XZ 1 "2020-02-01" "Tukaani" "XZ Utils"
. .
.SH NAME .SH NAME
xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
@ -1005,6 +1005,25 @@ instead of
until the details have been decided. until the details have been decided.
.RE .RE
.IP "" .IP ""
For 32-bit
.BR xz
there is a special case: if the
.I limit
would be over
.BR "4020\ MiB" ,
the
.I limit
is set to
.BR "4020\ MiB" .
(The values
.B 0
and
.B max
aren't affected by this.
A similar feature doesn't exist for decompression.)
This can be helpful when a 32-bit executable has access
to 4\ GiB address space while hopefully doing no harm in other situations.
.IP ""
See also the section See also the section
.BR "Memory usage" . .BR "Memory usage" .
.TP .TP