Re: slow transfers to a full-speed device

• Subject: Re: slow transfers to a full-speed device
• From: Eric Smith <email@hidden>
• Date: Wed, 15 Jun 2005 18:52:11 -0700
• Delivered-to: email@hidden
• Delivered-to: email@hidden

Eric

On Jun 15, 2005, at 8:31 AM, David Ferguson wrote:

Eric,

This is easy.

WritePipe() can take any size buffer -- we will set up the USB controller to send it in MaxPacketSize packets.

Regards,

David Ferguson
USB Software Team
Apple Computer

At 7:32 AM -0700 6/15/05, Eric Smith wrote:

Yes, it does seem a really bad way to send large amounts of data. Unfortunately, I don't know how to get around it. Bulk packet size is 64 bytes, and I've got about 6M bits all in a single byte buffer. The only function I see for reading/writing the bulk endpoint is ReadPipe()/WritePipe()... so I use that while incrementing the pointer to the buffer by 64. How do I remove the overhead? (I'm reading and writing to bulk endpoints on this device, where large amounts of data are involved).

Thanks,
Eric Smith

On Jun 14, 2005, at 11:03 PM, David Ferguson wrote:

This is a really inefficient way to send bytes to a device. Try using a much larger buffer to reduce the overhead per transfer. I would think you'll see a significant improvement if you use even 32k, but you should try transfers from 4k (the pagesize of MacOS X) to 128k. You may also want to make sure your data is aligned on a 64-byte boundary to put the maximum number of bytes per transfer descriptor (a USB controller data structure used inside the IOUSBFamily).

David Ferguson
Apple Computer

At 8:52 PM -0700 6/14/05, Eric Smith wrote:

I've got a full speed USB device, and am writing data in 64 byte packets to a bulk endpoint. This is just sitting in a for loop, doing a bunch of WritePipe() calls. My download speed over several megabits is only about 500Kbps. I'm using Tiger, and a dual 1.8 G5. I get about the same performance on a G4 iBook running Tiger. The same USB device gets about 6Mbps download on a windows-based machine. I did not write the windows code, so I don't know any of the details of that implementation.

Any ideas or suggestions?

Thanks,
Eric Smith

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