[U-Boot] [RFC V2 PATCH 0/3] Add cache for block devices

[Eric Nelson]

Hi Marek,

On 03/20/2016 06:59 PM, Marek Vasut wrote:
> On 03/21/2016 02:45 AM, Eric Nelson wrote:
>> Here's a more full-featured implementation of a cache for block
>> devices that uses a small linked list of cache blocks.
> 
> Why do you use linked list ? You have four entries, you can as well
> use fixed array. Maybe you should implement an adaptive cache would
> would use the unpopulated malloc area and hash the sector number(s)
> into that area ?
> 

I was looking for a simple implementation that would allow tweaking of
the max entries/size per entry.

We could get higher performance through hashing, but with such a
small cache, it's probably not worth extra code.

Using an array and re-allocating on changes to the max entries variable
is feasible, but I think it would be slightly more code.

>> Experimentation loading a 4.5 MiB kernel from the root directory of
>>  a FAT filesystem shows that a single cache entry of a single
>> block is the only
> 
> only ... what ? This is where things started to be interesting, but
> you leave us hanging :)
> 

Oops.

... I was planning on re-wording that.

My testing showed no gain in performance (additional cache hits) past a
single entry of a single block. This was done on a small (32MiB)
partition with a small number of files (~10) and only a single
read is skipped.

=> blkc c ; blkc i ; blkc 0 0 ;
changed to max of 0 entries of 0 blocks each
=> load mmc 0 10008000 /zImage
reading /zImage
4955304 bytes read in 247 ms (19.1 MiB/s)
=> blkc
block cache:
0	hits
7	misses
0	entries in cache
trace off
max blocks/entry 0
max entries 0
=> blkc c ; blkc i ; blkc 1 1 ;
changed to max of 1 entries of 1 blocks each
=> load mmc 0 10008000 /zImage
reading /zImage
4955304 bytes read in 243 ms (19.4 MiB/s)
=> blkc
block cache:
1	hits
6	misses
1	entries in cache
trace off
max blocks/entry 1
max entries 1

I don't believe that enabling the cache is worth the extra code
for this use case.

By comparison, a load of 150 MiB compressed disk image from
ext4 showed a 30x speedup with the V1 patch (single block,
single entry) from ~150s to 5s.

Without some form of cache, the 150s was long enough to make
a user (me) think something is broken.

Regards,


Eric