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SubjectRe: Minutes from Feb 21 LSE Call
Hanna Linder <hannal@us.ibm.com> wrote:
>
>
> Dave coded up an initial patch for partial object based rmap
> which he sent to linux-mm yesterday.

I've run some numbers on this. Looks like it reclaims most of the
fork/exec/exit rmap overhead.

The testcase is applying and removing 64 kernel patches using my patch
management scripts. I use this because

a) It's a real workload, which someone cares about and

b) It's about as forky as anything is ever likely to be, without being a
stupid microbenchmark.

Testing is on the fast P4-HT, everything in pagecache.

2.4.21-pre4: 8.10 seconds
2.5.62-mm3 with objrmap: 9.95 seconds (+1.85)
2.5.62-mm3 without objrmap: 10.86 seconds (+0.91)

Current 2.5 is 2.76 seconds slower, and this patch reclaims 0.91 of those
seconds.


So whole stole the remaining 1.85 seconds? Looks like pte_highmem.

Here is 2.5.62-mm3, with objrmap:

c013042c find_get_page 601 10.7321
c01333dc free_hot_cold_page 641 2.7629
c0207130 __copy_to_user_ll 687 6.6058
c011450c flush_tlb_page 725 6.4732
c0139ba0 clear_page_tables 841 2.4735
c011718c pte_alloc_one 910 6.5000
c013b56c do_anonymous_page 954 1.7667
c013b788 do_no_page 1044 1.6519
c015b59c d_lookup 1096 3.2619
c013ba00 handle_mm_fault 1098 4.6525
c0108d14 system_call 1116 25.3636
c0137240 release_pages 1828 6.4366
c013a1f4 zap_pte_range 2616 4.8806
c013f5c0 page_add_rmap 2776 8.3614
c0139eac copy_page_range 2994 3.5643
c013f70c page_remove_rmap 3132 6.2640
c013adb4 do_wp_page 6712 8.4322
c01172e0 do_page_fault 8788 7.7496
c0106ed8 poll_idle 99878 1189.0238
00000000 total 158601 0.0869

Note one second spent in pte_alloc_one().


Here is 2.4.21-pre4, with the following functions uninlined

pte_t *pte_alloc_one(struct mm_struct *mm, unsigned long address);
pte_t *pte_alloc_one_fast(struct mm_struct *mm, unsigned long address);
void pte_free_fast(pte_t *pte);
void pte_free_slow(pte_t *pte);

c0252950 atomic_dec_and_lock 36 0.4800
c0111778 flush_tlb_mm 37 0.3304
c0129c3c file_read_actor 37 0.2569
c025282c strnlen_user 43 0.5119
c012b35c generic_file_write 46 0.0283
c0114c78 schedule 48 0.0361
c0129050 unlock_page 53 0.4907
c0140974 link_path_walk 57 0.0237
c0116740 copy_mm 62 0.0852
c0130740 __free_pages_ok 62 0.0963
c0126afc handle_mm_fault 63 0.3424
c01254c0 __free_pte 67 0.8816
c0129198 __find_get_page 67 0.9853
c01309c4 rmqueue 70 0.1207
c011ae0c exit_notify 77 0.1075
c0149b34 d_lookup 81 0.2774
c0126874 do_anonymous_page 83 0.3517
c0126960 do_no_page 86 0.2087
c01117e8 flush_tlb_page 105 0.8750
c0106f54 system_call 138 2.4643
c01255c8 copy_page_range 197 0.4603
c0130ffc __free_pages 204 5.6667
c0125774 zap_page_range 262 0.3104
c0126330 do_wp_page 775 1.4904
c0113c18 do_page_fault 864 0.7030
c01052f8 poll_idle 6803 170.0750
00000000 total 11923 0.0087

Note the lack of pte_alloc_one_slow().

So we need the page table cache back.

We cannot put it in slab, because slab does not do highmem.

I believe the best way to solve this is to implement a per-cpu LIFO head
array of known-to-be-zeroed pages in the page allocator. Populate it with
free_zeroed_page(), grab pages from it with __GFP_ZEROED.

This is a simple extension to the existing hot and cold head arrays, and I
have patches, and they don't work. Something in the pagetable freeing path
seems to be putting back pages which are not fully zeroed, and I didn't get
onto debugging it.

It would be nice to get it going, because a number of architectures can
perhaps nuke their private pagetable caches.

I shall drop the patches in next-mm/experimental and look hopefully
at Dave ;)
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