Re: [LSF/MM TOPIC]swap improvements for fast SSD

[Shaohua Li <shli@kernel.org>]

On Sun, Feb 03, 2013 at 08:56:15PM -0800, Hugh Dickins wrote:
> On Sun, 27 Jan 2013, Shaohua Li wrote:
> > On Sat, Jan 26, 2013 at 01:40:55PM +0900, Kyungmin Park wrote:
> > > 5. SSD related optimization, mainly discard support.
> > > 
> > > Now swap codes are based on each swap slots. it means it can't
> > > optimize discard feature since getting meaningful performance gain, it
> > > requires 2 pages at least. Of course it's based on eMMC. In case of
> > > SSD. it requires more pages to support discard.
> > > 
> > > To address issue. I consider the batched discard approach used at filesystem.
> > > *Sometime* scan all empty slot and it issues discard continuous swap
> > > slots as many as possible.
> > 
> > I posted a patch to make discard async before, which is almost good to me,
> > though we still discard a cluster. 
> > http://marc.info/?l=linux-mm&m=135087309208120&w=2
> 
> Any reason why you point to 2012/10/22 patch rather than the 2012/11/19?
> 
> Seeing this reminded me to take your 1/2 and 2/2 (of 11/19) out again and
> give them a fresh run - though they were easier to apply to 3.8-rc rather
> than mmotm with your locking changes, so it was 3.8-rc6 I tried.
> 
> As I reported in private mail last year, I wish you'd remove the "buddy"
> from description of your 1/2 allocator, that just misled me; but I've not
> experienced any problem with the allocator, and I still like the direction
> you take with improving swap discard in 2/2.
> 
> This time around I've not yet seen any "swap_free: Unused swap offset entry"
> messages (despite forgetting to include your later SWAP_MAP_BAD addition to
> __swap_duplicate() - I still haven't thought that through to be honest),
> but did again get the VM_BUG_ON(error == -EEXIST) in __add_to_swap_cache()
> called from add_to_swap() from shrink_page_list().
> 
> Since it came after 1.5 hours of load, I didn't give it much thought,
> and just went on to test other things, thinking I could easily reproduce
> it later; but have failed to do so in many hours since.  Still trying.

Missed this mail, sorry. I'm planing to repost the patches against linux-next (because
of the locking changes) and will include the SWAP_MAP_BAD change. I did see
problems without the SWAP_MAP_BAD change.

Thanks,
Shaohua

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