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SubjectRe: swap on eMMC and other flash
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On Friday 06 April 2012, Minchan Kim wrote:
> On Sat, Mar 31, 2012 at 2:44 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>
> > We've had a discussion in the Linaro storage team (Saugata, Venkat and me,
> > with Luca joining in on the discussion) about swapping to flash based media
> > such as eMMC. This is a summary of what we found and what we think should
> > be done. If people agree that this is a good idea, we can start working
> > on it.
> >
> > The basic problem is that Linux without swap is sort of crippled and some
> > things either don't work at all (hibernate) or not as efficient as they
> > should (e.g. tmpfs). At the same time, the swap code seems to be rather
> > inappropriate for the algorithms used in most flash media today, causing
> > system performance to suffer drastically, and wearing out the flash
> > hardware
> > much faster than necessary. In order to change that, we would be
> > implementing the following changes:
> >
> > 1) Try to swap out multiple pages at once, in a single write request. My
> > reading of the current code is that we always send pages one by one to
> > the swap device, while most flash devices have an optimum write size of
> > 32 or 64 kb and some require an alignment of more than a page. Ideally
> > we would try to write an aligned 64 kb block all the time. Writing aligned
> > 64 kb chunks often gives us ten times the throughput of linear 4kb writes,
> > and going beyond 64 kb usually does not give any better performance.
> >
>
> It does make sense.
> I think we can batch will-be-swapped-out pages in shrink_page_list if they
> are located by contiguous swap slots.

But would that guarantee that all writes are the same size? While writing
larger chunks would generally be helpful, in order to guarantee that we
the drive doesn't do any garbage collection, we would have to do all writes
in aligned chunks. It would probably be enough to do this in 8kb or
16kb units for most devices over the next few years, but implementing it
for 64kb should be the same amount of work and will get us a little bit
further.

I'm not sure what we would do when there are less than 64kb available
for pageout on the inactive list. The two choices I can think of are
either not writing anything, or wasting the swap slots and filling
up the data with zeroes.

> > 2) Make variable sized swap clusters. Right now, the swap space is
> > organized in clusters of 256 pages (1MB), which is less than the typical
> > erase block size of 4 or 8 MB. We should try to make the swap cluster
> > aligned to erase blocks and have the size match to avoid garbage collection
> > in the drive. The cluster size would typically be set by mkswap as a new
> > option and interpreted at swapon time.
> >
>
> If we can find such big contiguous swap slots easily, it would be good.
> But I am not sure how often we can get such big slots. And maybe we have to
> improve search method for getting such big empty cluster.

As long as there are clusters available, we should try to find them. When
free space is too fragmented to find any unused cluster, we can pick one
that has very little data in it, so that we reduce the time it takes to
GC that erase block in the drive. While we could theoretically do active
garbage collection of swap data in the kernel, it won't get more efficient
than the GC inside of the drive. If we do this, it unfortunately means that
we can't just send a discard for the entire erase block.

Arnd


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