Re: [PATCH/RFC] ps3/block: Add ps3vram-ng driver for accessing video RAM as block device

On Fri, Mar 06 2009, Geert Uytterhoeven wrote:

On Fri, 6 Mar 2009, Jens Axboe wrote:

quoted

On Fri, Mar 06 2009, Geert Uytterhoeven wrote:

quoted

On Fri, 6 Mar 2009, Jens Axboe wrote:

quoted

On Thu, Mar 05 2009, Geert Uytterhoeven wrote:

quoted

But then I noticed ps3vram_make_request() may be called concurrently,
so I had to add a mutex to avoid data corruption. This slows the
driver down, and in the end, the version with a thread turns out to be
ca. 1% faster. The version without a thread is about 50 lines less
code, though.

That is correct, ->make_request_fn may get reentered. I'm not surprised
that performance dropped if you just shoved everything under a mutex.
You could be a little more smart and queue concurrent bio's for
processing when the current one is complete though, there are several
approaches there that be a lot faster than going all the way through the
IO stack and scheduler just to avoid concurrency.

Yes, using a spinlock and queueing requests on a list if the driver is
busy can be done after 2.6.29...

Certainly. Even just replacing your current mutex with a spinlock during
the memcpy() would surely be a lot faster. Or even just grabbing the
mutex before calling into the write for the duration of the bio. The way
you do it is certain context switch death :-)

It's not just the memcpy(). ps3vram_{up,down}load() call msleep(), so
I cannot use a spinlock.

Ah right, I hadn't looked close enough. But putting the mutex_lock()
outside of the bio_for_each_segment() is going to be much faster than
getting/releasing it for each segment.

-- 
Jens Axboe