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SubjectRE: swap on eMMC and other flash
Hi Arnd,

Regarding time to issue discard/TRIM commands:
It would be advised to issue the discard command immediately after deleting/freeing a SWAP cluster (i.e. as soon as it becomes available).

Regarding SWAP page size:
Working with as large as SWAP pages as possible would be recommended (preferably 64KB). Also, writing in a sequential manner as much as possible while swapping large quantities of data is also advisable.

SWAP pages and corresponding transactions should be aligned to the SWAP page size (i.e. 64KB above), the alignment should correspond to the physical storage "LBA 0", i.e. to the first LBA of the storage device (and not to a logical/physical partition).

Thanks,
Alex

> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@arndb.de]
> Sent: Monday, April 02, 2012 5:55 PM
> To: Hugh Dickins
> Cc: linaro-kernel@lists.linaro.org; Rik van Riel; linux-
> mmc@vger.kernel.org; Alex Lemberg; linux-kernel@vger.kernel.org; Luca
> Porzio (lporzio); linux-mm@kvack.org; Hyojin Jeong; kernel-
> team@android.com; Yejin Moon
> Subject: Re: swap on eMMC and other flash
>
> On Monday 02 April 2012, Hugh Dickins wrote:
> > On Mon, 2 Apr 2012, Arnd Bergmann wrote:
> > >
> > > Another option would be batched discard as we do it for file
> systems:
> > > occasionally stop writing to swap space and scanning for areas that
> > > have become available since the last discard, then send discard
> > > commands for those.
> >
> > I'm not sure whether you've missed "swapon --discard", which switches
> > on discard_swap_cluster() just before we allocate from a new cluster;
> > or whether you're musing that it's no use to you because you want to
> > repurpose the swap cluster to match erase block: I'm mentioning it in
> > case you missed that it's already there (but few use it, since even
> > done at that scale it's often more trouble than it's worth).
>
> I actually argued that discard_swap_cluster is exactly the right thing
> to do, especially when clusters match erase blocks on the less capable
> devices like SD cards.
>
> Luca was arguing that on some hardware there is no point in ever
> submitting a discard just before we start reusing space, because
> at that point it the hardware already discards the old data by
> overwriting the logical addresses with new blocks, while
> issuing a discard on all blocks as soon as they become available
> would make a bigger difference. I would be interested in hearing
> from Hyojin Jeong and Alex Lemberg what they think is the best
> time to issue a discard, because they would know about other hardware
> than Luca.
>
> Arnd

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