Hi,
I may propose that read data done by 64kb block. So to read first data 63-64
the driver must read 0-64 and then cache 0-63 for other use.
The same for tail data. The driver read more then needed and redundant data is
cached.
The is no full cache mechanism to speedup data and cache size is small.
Enjoy.
On 19.12.2014, at 6:33, Sheldon Lu wrote:
> Hi All,
>
> Is there any one can help answer my question ?
> I really need to understand why and explain this to my custmer.
> Please share your ideas or give me some clues if any one knows the answer.
>
> Really thank you!
>
> Sheldon
>
> From: Sheldon Lu [mailto:lxd5...@gmail.com]
> Sent: Thursday, December 18, 2014 10:32 AM
> To: edk2-devel@lists.sourceforge.net
> Cc: jljus...@intel.com; ruiyu...@intel.com; GaryLi(李俊); KasalinYi(易花中);
> RuiDai(戴潇睿)
> Subject: a question about cache mechanism in EnhancedFatDxe
>
> Hi guys,
>
> I am studying the EnhancedFatDxe driver recently. I met a question when
> looking into FatAccessCache() in DiskCache.c. As the "Routine Description"
> says:
> "2. Access of Data cache (CACHE_DATA):
> The access data will be divided into UnderRun data, Aligned data and OverRun
> data;
> The UnderRun data and OverRun data will be accessed by the Data cache,
> but the Aligned data will be accessed with disk directly."
>
> I'm very confused with "the Aligned data will be accessd with disk directly".
> I can't understand why it only caches "unaligned" data but exclude the
> possiblity to cache "aligned" data ? Does it discount the performance ?
>
> An example to show how it works:
>
> If I want to read the data from offset 63KB to offset 129KB(these
> offsets are relative to the Root directry). As the cache alignment is 64KB in
> FAT Data Region, so the data is divided into 3 parts:
>
> 63KB~64KB … 64KB~128KB … 128KB~129KB
>
> The current cache mechanism of EnhancedFatDxe only caches the accesses
> to “63KB~64KB”and “128KB~129KB”,but let the access to “64KB~128KB”goes to
> disk directly!
>
> I’m very confused why it only caches the head and tail but leaves the main
> body behind ?
>
> Any one knows why it is designed like this ? Isn’t it better to cache ALL
> datas without discrimination ?
>
> Looking forward to your reply!
> Thank you so much!
>
> B.R.
>
> Sheldon
>
>
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