linux-kernel - USB performance bug since kernel 2.6.13 (CRITICAL???)

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Message-ID: <20061012192156.97357.qmail@web58106.mail.re3.yahoo.com>
Date:	Thu, 12 Oct 2006 12:21:56 -0700 (PDT)
From:	Open Source <opensource3141@...oo.com>
To:	linux-kernel@...r.kernel.org, linux-usb-devel@...rceforge.net
Subject: USB performance bug since kernel 2.6.13 (CRITICAL???)

Hi all, 

I am  writing regarding a performance issue that I recently observed after upgrading from kernel 2.6.12 to 2.6.17.  I did some hunting around and have found that the issue first arises in 2.6.13.

I am using a device that submits URBs asynchronously using the libusb devio infrastructure.  In version 2.6.12 I am able to submit and reap URBs for my particular application at a transaction rate of one per millisecond.  A transaction consists of a single WRITE URB (< 512 bytes) followed by a single READ URB (1024 bytes).  Once I upgrade to version 2.6.13, the transactional rate drops to one per 4 milliseconds!

The overall performance of a particular algorithm is increased from a total execution time of 75 seconds to over 160 seconds.  The only difference between the two tests is the kernel.  Microsoft Windows executes the algorithm in 70-75 seconds!

I am using a Fedora Core distribution with FC4 kernels for testing.  Is there some new incantation that is required in my user-mode driver to get around a "feature" in recent kernels?  Does anyone else know about this?  I was not able to easily find discussion about this on the newsgroups.  It appears that this problem has been around for a while, if it is indeed a problem.

I am not a subscriber to the linux-kernel mailing list but have cross-posted to it since this seems like a serious enough issue.  Please continue to keep any responses on linux-usb-devel as well so I can see them in my email box.

Thank you,
Beleaguered Open Source Fan

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