Scheduler question

[Daniel O'Connor]

Hi,
I am writing a program which reads from a data acquisition chassis connected to a radar via USB. The interface is a Cypress FX2 and I am communicating via libusb.

The program starts a thread which sits in a loop doing nothing but libusb_handle_events_timeout() which in turn runs a callback if a transfer is complete. Each transfer is in a struct which has a mutex and a 'done' flag (the former protects the later) which is set when the callback is run by libusb.

The main thread sits in a loop waiting for the next transfer to be done and when it is copying data out to be further processed and then written out to disk and/or another process for some further mangling.

After each USB transfer is done with (ie the main thread has passed it all out for further processing) the main thread re-submits it to libusb.

I only have about 10 milliseconds of buffering (96kbyte FIFO, 8Mbyte/sec) in the hardware, however I have about 128Mb of USB requests queued up to libusb. hps@ informed me that libusb will only queue 16kbyte (2msec) in the kernel at one time although I have increased this.

I hooked up a logic analyser and I can see most of the time it's fairly regularly transferring 16k of data every 2msec.

If I load up the disk by, eg, tar -cf /dev/null /local0 I find it drops out and I can see gaps in the transfers until eventually the FIFO fills up and it stops.

I am wondering if this is a scheduler problem (or I am expecting too much :) in that it is not running my libusb thread reliably under load. The other possibility is that it is a USB issue, although I am looking at using isochronous transfers instead of bulk.

I just noticed that the USB controller and ATA controller share an IRQ, but I wouldn't expect that to cause a problem..

This is running on FreeBSD 8.1-STABLE, Core 2 Duo with ICH9 chipset.

Thanks.

--
Daniel O'Connor software and network engineer
for Genesis Software - http://www.gsoft.com.au
"The nice thing about standards is that there
are so many of them to choose from."
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