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

[Marek Vasut]

On 03/21/2016 06:56 PM, Eric Nelson wrote:
> Hi Marek,

Hi!

> On 03/21/2016 09:49 AM, Marek Vasut wrote:
>> On 03/21/2016 02:48 PM, Eric Nelson wrote:
>>> 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.
>>
>> The hashing function can be a simple modulo on sector number ;-) That'd
>> be less code than linked lists.
>>
> 
> I'm not seeing how.
> 
> I'm going look first at a better way to integrate than the approach
> taken in patch 3.

I will dive in the code itself and comment if applicable.

>>> Using an array and re-allocating on changes to the max entries variable
>>> is feasible, but I think it would be slightly more code.
>>
>> That would indeed be 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.
>>
>> I'd kinda expect that indeed.
>>
> 
> Yeah, and the single-digit-ms improvement isn't worth much.
> 
>>> => 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.
>>
>> I'm obviously loving this improvement.
>>
> 
> Glad to hear it.
> 
:)

-- 
Best regards,
Marek Vasut