Re: Speed of SDHCI subsystem

[Pierre Ossman <drzeus-list@drzeus.cx>]

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On Tue, 19 May 2009 00:37:50 +0200
"J.A. Magallón" <jamagallon@ono.com> wrote:

> Hi all...
> 
> I have notice something strange with SD cards.
> I have an Aspire One and had an 8Gb SDHC card that used for my home,
> and have recently upgraded to a 16Gb one.
> This new one came with an USB reader.
> 
> The thing is that cards look much faster when accessed through the USB
> key that directly on the readers of the One.
> 
> Some numbers with hdparm:
> 
> TakeMS 8Gb Class 6:
> 	Gives 10MB/s on the slot, 17MB/s via the USB key
> SanDisk UltraII 16Gb, Class 4, advertised as 15Mb/s:
> 	Gives 10MB/s on the slot, 14MB/s on USB
> 
> I know that hdparm is not a benchmark, but I supposed it should be
> limited by the media, not the connection.
> 
> But somehow the SDHCI/MMC subsystem seems to be stuck at 10MB/s, independent
> of the quality of the media.
> 
> Any ideas ?
> 

Given your numbers I'd guess that your USB reader supports high-speed
and your built-in one does not. The theoretical throughput without
high-speed is 12.5 MB/s (SI-prefix). With some overhead, and the fact
that many controllers have a lower maximum frequency than 25 MHz, makes
your 10 MB/s pretty reasonable.

For reference, I've managed to achieve ~23 MB/s using a Sandisk Extreme
III card and a JMicron controller, so there doesn't seem to be any
major inherent bottle necks in the MMC stack.

Rgds
-- 
     -- Pierre Ossman

  Linux kernel, MMC maintainer        http://www.kernel.org
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