Recently in Computer Category

Otherwise known as "calculating a long list of numbers then adding them all up".  For a GPGPU (OpenCL) simulation program at work, I needed to calculate around 160 numbers which would be averaged to produce one result for storage in a 1024x1024 element array.  That's 160 numbers for each of 1024x1024 pixels, which would be a lot to store as an intermediate result for a later step of averaging on the GPU, or (heaven forbid) to be copied back to system memory.

This is very cool.  Volunteer contributors used satellite imagery, which was made available especially, to create detailed maps of Haiti which the aid workers used to help find their way.

I've just spent a few hours rearranging my Git repositories to use Gitolite, something I've been meaning to do for a long time. This gives me much better control over access permissions, potentially letting me give other people access to my repository without handing over shells and/or real user accounts.

So, I'm now the proud owner of a Lenovo Thinkpad T500 with WSXGA+ screen (1680x1050), 7200rpm HDD, Core 2 Duo, UK keyboard and 9-cell battery. Exactly to specification, and within budget :D

{I am getting} {scary messages} in {curly brackets} in my laptop's {dmesg}.  The universal warning signal for imminent hard drive death and data loss.  I get dropouts of about 30 seconds at a time with no hard drive activity (before the kernel realises and reset the link), during which time the computer is mostly frozen (no HDD I/O possible), and this seems to be happening more and more often.  In addition, the power connector is broken - the central pin in the laptop's connector snapped off.  Since the pin stays fixed in the hole in the adaptor's plug, it still just about works if it's carefully pushed in and the cable wrapped round to put pressure in the right way.  However, I don't know how long either of these will hold out.

Of course I'm backed up to the hilt with distributed version control, so I'm not in immediate danger of losing anything particularly important.  However, it's apparent that I'll need to buy a new laptop in the near future.  At the moment I'm looking at a Lenovo Thinkpad T500 with WSXGA+ (1680x1050) screen and Radeon graphics, but does anyone have any other suggestions?  My non-negotiable requirements are:

• Linux-friendly wifi and graphics.
  
• Dual core, or at least HT.  This really does make a huge difference.
  
• Widescreen.  1680x1050 with a 15.4" screen gives a resolution I like.
• UK keyboard layout (i.e. UK market, ideally with delivery to Germany possible).
• DVD drive.
  

Ideally it would also have:

• Decent battery life, or the possibility to buy spare or larger batteries during the next few years once the original one becomes a plastic box of jelly.
• VGA output.
• Fast-ish hard drive (7200rpm or higher.  I'm not sure how much of a difference this makes, but I do a lot of compiling and so on.  No need to go overboard with solid-state disks for hundreds of extra pounds.
  

I'm not too bothered about:

• Bluetooth (I don't use it at the moment).
• Huge hard drive - I get on fine with only about 80Gb at the moment.

Any suggestions on a comment or email to this address..

While debugging something different late last night, I noticed some flags in one of Glamo's registers which looked interesting: FIFO settings for the LCD engine.  This reminded me of an observation by Lars a few weeks ago that the LCD engine seems to conflict with Glamo's 2D engine on memory accesses, leading to slower performance of accelerated 2D operations when the screen is switched on.  So I turned the FIFO up to "8 stages" (from 1) to see what happened.  The result was much faster 2D operations - literally twice the speed!

At "8 stages", the price of this was that the display became jittery and unstable.  However, the same speed improvement is seen at the "4 stages" setting. I've also seen some occasional artifacts with this setting, so I'm using 2 stages at the moment, where the speed is still right up there.  I'll be testing some more and seeing if things can be tuned even more.

Because we don't make the maximum use possible of the 2D engine, this doesn't immediately translate into a huge increase in the UI speed.  But the differences are very obvious with x11perf and some of my test programs. The program I showed in the screenshot recently jumped from 45-48fps right up to 95-98fps!

When the CPU needs to do something which depends on a result which the GPU is currently working on, it has to wait for the GPU to catch up.  One of the biggest problems with the current architecture of xf86-video-glamo, both DRM and non-DRM versions, is that they do this waiting by spinning in a tight loop, each time checking the current status of the GPU, until it's caught up.  This isn't great for a few reasons.  It makes no use of the parallelism between the CPU and the GPU, so precious CPU time is being wasted while something more useful could be being done.  If there's nothing else to do, then the CPU could be sleeping - reducing power consumption.

Most GPUs, including Glamo, have a mechanism for being a little smarter.  The kernel can ask the chip to trigger an interrupt when a certain point in the command queue has been reached.  When a process needs to wait, the kernel can send it to sleep and watch out for the interrupt.  When it happens, the process can be quickly woken back up in a low-latency fashion, meaning that the process gets back to work with very little latency.

This week, I've been implementing this kind of thing for the Glamo DRM driver.  It goes a bit like this:

• Process submits some rendering commands via one of the command submission ioctls.
• Kernel driver places rendering commands on Glamo's command queue.
  
• Process needs to wait for the GPU to catch up, so calls the wait ioctl.
• Kernel driver puts an extra sequence of commands, called a fence, onto the command queue.  A unique number is associated with the fence.  The number is recorded by the kernel.
• When the GPU processes the fence, it raises the interrupt and places a unique number into a certain register.
  
• The interrupt handler checks this number, and wakes up the corresponding process.
  

I wrote a test program which tells Glamo to fill the whole screen with colour as fast as it can, waiting for the GPU to catch up each time.  The task was to make the program run with close to zero CPU usage while still getting the full framerate that I could get using busywaits.  The task was achieved successfully, and here's a screenshot to prove it.  The framerate - just below 50fps when doing fills of the entire VGA screen - was exactly the same with busywaits.  It even went up a little (to 50-51fps) when I improved the interrupt handling.

Things aren't always so great.  When the command sequence to be executed is very short, the overheads of fencing and scheduling become significant, and the overall rate drops.  However, it shouldn't be too difficult to design some kind of heuristic to use busywaits as a low-latency strategy in such cases.

There are still a few problems to iron out.  The fence mechanism seems to be able to fall out of sync with things, leading to processes waiting for too long (or even forever).  But when it works, some things do seem to feel a little faster in general use.

Geeks may be interested in the actual code.

Having realised that most of the bugs I was chasing aren't actually my fault (see the to-do list), I've been allowing myself to work on the Mesa driver for Glamo.  A rebase (sorry!) against the latest Git master branch of upstream Mesa was required to get some important DRI2 fixes, but now it works.  I'm not going to go into much detail (I need to sleep), but enough of the initialisation and buffering stuff works that things can be drawn (using Glamo's 2D engine at the moment) and then put on the screen (front buffer) successfully.  It's enough to draw a blue rectangle at the moment.  Not much, but you wouldn't believe how much Stuff has to be working correctly for that to happen.

Actually, I expect that readers of the journal would believe exactly how much Stuff is involved...

I've done a backport of the BFS scheduler to the 2.6.29 kernel currently used by Openmoko.  This is only for people who want to try something new and exciting before the official release of the forthcoming 2.6.31 kernel for Openmoko with our new repository layout (at which point the upstream BFS patch should apply cleanly).

It's hardly tested, and I might have screwed up the patch completely (I don't really know my way around these areas of the kernel), but here it is: BFS for Openmoko.

My experience was, despite reports of dramatic speedups on Android, that there wasn't a huge amount of difference compared to the conventional scheduler.  Your experiences may differ, however.