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

[Marek Vasut]

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 at denx.de>
>>>>>> Cc: Tom Rini <trini at konsulko.com>
>>>>>> Cc: Simon Glass <sjg at chromium.org>
>>>>>
>>>>> Reviewed-by: Tom Rini <trini at 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.

>>> 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 ?

-- 
Best regards,
Marek Vasut