[U-Boot] [PATCH] [RFC] blk: Increase cache element size

[Tom Rini <trini@konsulko.com>]

On Thu, Aug 16, 2018 at 01:42:33PM +0200, Marek Vasut wrote:
> On 08/15/2018 06:27 PM, Tom Rini wrote:
> > On Wed, Aug 15, 2018 at 06:20:16PM +0200, Marek Vasut wrote:
> >> On 08/15/2018 06:12 PM, Tom Rini wrote:
> >>> On Wed, Aug 15, 2018 at 06:04:50PM +0200, Marek Vasut wrote:
> >>>
> >>>> On 08/15/2018 04:30 PM, Tom Rini wrote:
> >>>>> On Wed, Aug 08, 2018 at 01:20:29PM +0200, Marek Vasut wrote:
> >>>>>
> >>>>>> Cache up to 4 kiB entries. 4 kiB is the default block size on ext4, yet
> >>>>>> the underlying block layer devices usually report support for 512B . In
> >>>>>> most cases, the 512B support is emulated (ie. SD cards, SSDs, USB sticks
> >>>>>> etc.) and the real block size of those devices is much bigger.
> >>>>>>
> >>>>>> To avoid performance degradation with such devices and FS setup, bump
> >>>>>> the maximum cache entry size to 4 kiB.
> >>>>>>
> >>>>>> Signed-off-by: Marek Vasut <marex@denx.de>
> >>>>>> Cc: Tom Rini <trini@konsulko.com>
> >>>>>> Cc: Simon Glass <sjg@chromium.org>
> >>>>>
> >>>>> Reviewed-by: Tom Rini <trini@konsulko.com>
> >>>>>
> >>>>> I'll pick this up post v2018.09 if no one objects, thanks!
> >>>>
> >>>> I object. I was hoping there'd be some discussion on how to solve this
> >>>> in a future-proof manner ... it's only a matter of time until someone
> >>>> uses ext4 with 8k blocks on an SSD ...
> >>>
> >>> In general, sure?  In specific, mkfs.ext4 1.42.13 man page says 1/2/4KiB
> >>> are the only valid values of block size, and based on having to whack
> >>> this for some other projects it's pretty common for OpenEmbedded at
> >>> least to spit out 1KiB block size images.
> >>
> >> OE spits 4k , that's how I triggered it,
> >> meta/classes/image_types.bbclass:EXTRA_IMAGECMD_ext2 ?= "-i 4096"
> >> meta/classes/image_types.bbclass:EXTRA_IMAGECMD_ext3 ?= "-i 4096"
> >> meta/classes/image_types.bbclass:EXTRA_IMAGECMD_ext4 ?= "-i 4096"
> > 
> > That's bytes-per-inode, I was referring to block size which is -b and
> > dynamic unless specified.
> 
> Right. Although, I think we mostly care about caching the inodes, not
> blocks, since the inodes are accessed repeatedly.

OK, so it's the inode block.

> >>> So unless you know of cases
> >>> today (or tomorrow, but not next year) where 8KiB is common or likely,
> >>> we should probably just bump this for now and maybe make it a tunable so
> >>> it's easily changed?
> >>
> >> It is already tunable, see blkcache config in blkcache command.
> >>
> >> But what I'd like to see is somehow the FS and the underlying storage
> >> negotiating the best settings. Can we get the FS block size from the
> >> block cache perspective ?
> > 
> > Good questions that I don't have an answer to.
> 
> What we can do is extend fs_devread() and blk_dread() with a new flag,
> bool cachable, to allow FS and other upper layers which read block
> devices to mark blocks that should specifically be cached as such. The
> block cache would then cache the block no matter what size it has.
> 
> The FS implementations should know which data should be cached because
> they will be accessed repeatedly and which data are not, so they can set
> the flag accordingly.
> 
> The other users of blk_dread() would need to be examined. Possibly the
> best solution right now to avoid problems would be to apply this patch
> first. Then, second, add the flag to blk_dread() and set it accordingly
> where applicable AND make the block cache cache blocks which are either
> <= 8 kiB OR with cachable flag. Then finally, probably in next release,
> drop the <= 8 kiB condition and cache only blocks which are flagged as
> cachable to avoid polluting the block cache with useless small crumbs of
> data.
> 
> The downside is, the blk_dread() would look a bit asymmetrical compared
> to the other functions with the cachable flag in it's parameters.
> 
> Thoughts ?

Frankly, this sounds rather complicated when we might just need to
increase the size of what we can cache a bit.  We aren't a long running
facility and so long as it's tunable, it's also most likely being used
in machines where we have enough DDR that we can a few more kilobytes.

-- 
Tom
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