Re: [PATCH] lazy freeing of memory through MADV_FREE
From: Nick Piggin
Date: Mon Apr 23 2007 - 06:22:03 EST
Rik van Riel wrote:
Use TLB batching for MADV_FREE. Adds another 10-15% extra performance
to the MySQL sysbench results on my quad core system.
Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
---
Rik van Riel wrote:
I've added a 5th column, with just your mmap_sem patch and
without my madv_free patch. It is run with the glibc patch,
which should make it fall back to MADV_DONTNEED after the
first MADV_FREE call fails.
With the attached patch to make MADV_FREE use tlb batching, not
only do we gain an additional 10-15% performance but Nick's
mmap_sem patch also shows the performance increase that we
expected to see.
It looks like the tlb flushes (and IPIs) from zap_pte_range()
could have been the problem. They're gone now.
I guess it is a good idea to batch these things. But can you
do that on all architectures? What happens if your tlb flush
happens after another thread already accesses it again, or
after it subsequently gets removed from the address space via
another CPU?
The second column from the right has Nick's patch and my own
two patches. Performance with 16 threads is almost triple what
it used to be...
vanilla glibc glibc glibc glibc glibc glibc
madv_free madv_free madv_free madv_free
mmap_sem mmap_sem mmap_sem
tlb batch tlb_batch
threads
1 610 609 596 545 534 547 537
2 1032 1136 1196 1200 1180 1293 1194
4 1070 1128 2014 2024 2027 2248 2040
8 1000 1088 1665 2087 2089 2314 1869
16 779 1073 1310 1999 2012 2214 1557
Now that I think about it - this is all with the rawhide kernel
configuration, which has an ungodly number of debug config
options enabled.
I should try this with a more normal kernel, on various different
systems.
This is for another day. :)
First some ebizzy runs...
------------------------------------------------------------------------
--- linux-2.6.20.x86_64/mm/memory.c.orig 2007-04-23 02:48:36.000000000 -0400
+++ linux-2.6.20.x86_64/mm/memory.c 2007-04-23 02:54:42.000000000 -0400
@@ -677,11 +677,15 @@ static unsigned long zap_pte_range(struc
remove_exclusive_swap_page(page);
unlock_page(page);
}
- ptep_clear_flush_dirty(vma, addr, pte);
- ptep_clear_flush_young(vma, addr, pte);
SetPageLazyFree(page);
if (PageActive(page))
deactivate_tail_page(page);
+ ptent = *pte;
+ set_pte_at(mm, addr, pte,
+ pte_mkclean(pte_mkold(ptent)));
+ /* tlb_remove_page frees it again */
+ get_page(page);
+ tlb_remove_page(tlb, page);
continue;
}
}
--
SUSE Labs, Novell Inc.
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