Re: [patch 45/99] mm: unmapped page migration avoid unmap+remap overhead

[Davidlohr Bueso <dave@stgolabs.net>]

On Fri, 2014-12-12 at 16:56 -0800, akpm@linux-foundation.org wrote:
> From: Hugh Dickins <hughd@google.com>
> Subject: mm: unmapped page migration avoid unmap+remap overhead
> 
> Page migration's __unmap_and_move(), and rmap's try_to_unmap(), were
> created for use on pages almost certainly mapped into userspace.  But
> nowadays compaction often applies them to unmapped page cache pages: which
> may exacerbate contention on i_mmap_rwsem quite unnecessarily, since
> try_to_unmap_file() makes no preliminary page_mapped() check.
> 
> Now check page_mapped() in __unmap_and_move(); and avoid repeating the
> same overhead in rmap_walk_file() - don't remove_migration_ptes() when we
> never inserted any.
> 
> (The PageAnon(page) comment blocks now look even sillier than before, but
> clean that up on some other occasion.  And note in passing that
> try_to_unmap_one() does not use a migration entry when PageSwapCache, so
> remove_migration_ptes() will then not update that swap entry to newpage
> pte: not a big deal, but something else to clean up later.)
> 
> Davidlohr remarked in "mm,fs: introduce helpers around the i_mmap_mutex"
> conversion to i_mmap_rwsem, that "The biggest winner of these changes is
> migration": a part of the reason might be all of that unnecessary taking
> of i_mmap_mutex in page migration; 

Yeah, this is making a lot of sense.

> and it's rather a shame that I didn't
> get around to sending this patch in before his - this one is much less
> useful after Davidlohr's conversion to rwsem, but still good.

Now that I have some free hardware, I did some testing to consider this
patch for some SLE kernels (which still has the i_mmap mutex), and it
sure relieves a lot of the overhead/contention. On a 60-core box with a
file server benchmark we increase throughput by up to 60-70%:

new_fserver-61     21456.59 (  0.00%)    35875.59 ( 67.20%)
new_fserver-121    22335.16 (  0.00%)    38037.28 ( 70.30%)
new_fserver-181    23280.22 (  0.00%)    39518.54 ( 69.75%)
new_fserver-241    23194.88 (  0.00%)    39065.85 ( 68.42%)
new_fserver-301    23135.30 (  0.00%)    38464.88 ( 66.26%)
new_fserver-361    22922.97 (  0.00%)    38115.74 ( 66.28%)
new_fserver-421    22841.84 (  0.00%)    37859.06 ( 65.74%)
new_fserver-481    22643.83 (  0.00%)    37751.59 ( 66.72%)
new_fserver-541    22620.21 (  0.00%)    37036.09 ( 63.73%)
new_fserver-601    22593.85 (  0.00%)    36959.11 ( 63.58%)
new_fserver-661    22434.81 (  0.00%)    36629.28 ( 63.27%)
new_fserver-721    22219.68 (  0.00%)    36128.16 ( 62.60%)
new_fserver-781    22134.90 (  0.00%)    35893.50 ( 62.16%)
new_fserver-841    21901.59 (  0.00%)    35826.33 ( 63.58%)
new_fserver-901    21911.80 (  0.00%)    35285.66 ( 61.03%)
new_fserver-961    21810.72 (  0.00%)    35253.62 ( 61.63%)

Anyway, it's already picked up by Linus, but thought it would be nice to
have actual data.

Thanks,
Davidlohr