If we are using 1TB segments and we are allowed to randomise the heap, we can
put it above 1TB so it is backed by a 1TB segment. Otherwise the heap will be
in the bottom 1TB which always uses 256MB segments and this may result in a
performance penalty.
This functionality is disabled when heap randomisation is turned off:
echo 1 > /proc/sys/kernel/randomize_va_space
which may be useful when trying to allocate the maximum amount of 16M or 16G
pages.
On a microbenchmark that repeatedly touches 32GB of memory with a stride of
256MB + 4kB (designed to stress 256MB segments while still mapping nicely into
the L1 cache), we see the improvement:
Force malloc to use heap all the time:
# export MALLOC_MMAP_MAX_=0 MALLOC_TRIM_THRESHOLD_=-1
Disable heap randomization:
# echo 1 > /proc/sys/kernel/randomize_va_space
# time ./test
12.51s
Enable heap randomization:
# echo 2 > /proc/sys/kernel/randomize_va_space
# time ./test
1.70s
Signed-off-by: Anton Blanchard <an...@samba.org>
---
I've cc-ed Mel on this one. As you can see it definitely helps the base
page size performance, but I'm a bit worried of the impact of taking away
another of our 1TB slices.
Index: linux.trees.git/arch/powerpc/kernel/process.c
===================================================================
--- linux.trees.git.orig/arch/powerpc/kernel/process.c 2009-09-17
15:47:46.000000000 +1000
+++ linux.trees.git/arch/powerpc/kernel/process.c 2009-09-17
15:49:11.000000000 +1000
@@ -1165,7 +1165,22 @@ static inline unsigned long brk_rnd(void
unsigned long arch_randomize_brk(struct mm_struct *mm)
{
- unsigned long ret = PAGE_ALIGN(mm->brk + brk_rnd());
+ unsigned long base = mm->brk;
+ unsigned long ret;
+
+#ifdef CONFIG_PPC64
+ /*
+ * If we are using 1TB segments and we are allowed to randomise
+ * the heap, we can put it above 1TB so it is backed by a 1TB
+ * segment. Otherwise the heap will be in the bottom 1TB
+ * which always uses 256MB segments and this may result in a
+ * performance penalty.
+ */
+ if (!is_32bit_task() && (mmu_highuser_ssize == MMU_SEGSIZE_1T))
+ base = max_t(unsigned long, mm->brk, 1UL << SID_SHIFT_1T);
+#endif
+
+ ret = PAGE_ALIGN(base + brk_rnd());
if (ret < mm->brk)
return mm->brk;
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