WizBlog

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

2009 was an awful lot better than 2008 for me, for many reasons.  2008 had a lot of negative things of various types, but I won't go into detail about those. 2009 began with my falling into what could possibly be described as my dream job (particle accelerators and computers!), the completion of my PhD thesis and passing of the subsequent examination before being whisked off my feet by the beginning of an international adventure.  I suspect the adventure will be ongoing for the rest of my life.  When I moved out of my last long-term accommodation in Cambridge back in May, I knew that things would never be the same again.

{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..

On Friday I got back to Hamburg from San Francisco, where I'd been taking part in experiments at the Stanford Linear Accelerator Cente.  We used the LCLS to zap various kinds of object and measure the resulting signals.  Without going into too many gory details, I'm happy to be able to say that the experiment was a great success and we got several terabytes of useful data, which we'll be analysing for many months to come.

I hardly had any time to escape the campus or to explore, but I did manage to find some sites of geeky interest.  Here's a photo of the research stations at the end of the two mile long linear accelerator, which is hidden by the trees at the back of the picture.  The large concrete building on the right is End Station A, where the first experimental evidence for the existence of quarks was recorded around 1966.  Their experiment was like a much larger version of Rutherford's scattering experiment with alpha particles and gold foil.  Today, End Station A contains test experiments to prepare for the International Linear Collider.  I wasn't able to go inside - it's probably possible, subject to the particle accelerator's beam being directed elsewhere and talking to the right people, but there wasn't time and I wasn't keen to push the limits of my security pass (I also couldn't find the door..).

The similar concrete monstrosity on the left is End Station B, which probably contains similarly cool things (remember what I wrote about concrete?).  The long building which comes out between and runs off the picture to the left is the LCLS beam transport hall.  Standing where I took this picture, below me and to the left would be the undulator hall, which contains magnets which cause the electron beam to emit X-ray pulses of unimaginable brightness and brevity.  The tunnel emerges on the other side of the hill I'm standing on, where the electrons and X-rays move into the building where we worked.  The electron beam is dumped (basically by directing it into the ground) and the X-rays make their way into our experiment.

Due to popular demand, I've enabled comments and trackbacks here.  Please don't all spam me at once... :)

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.

Concrete blocks. Absolutely everywhere. That's my dominant impression of this international particle accelerator research centre after the first month. Concrete blocks shielding the outside world from radiation emitted by the shiny things hiding behind them. And generally, the bigger the pile of concrete blocks, the cooler the thing that's lurking behind.

Here are some photos from today's open day at DESY. Most of the things shown (everything apart from FLASH and XFEL) have nothing to do with what I work on, but they're still exciting to look at. The HERA and PETRA tunnels aren't normally open, least of all to the public, and there probably won't be another opportunity to see them for years. In pictures 38, 40, 42, 45, 46 and 51, you can see the sequence of bits of pipes and coils which guided electrons from PETRA, physically above HERA, into HERA's electron ring. HERA was switched off in September 2007, but almost all of it is still in the tunnels. You can also see wider views of the machine. The cylindrical pipe thing on the top is the superconducting ring of magnets which guided protons, and the pink boxy thing underneath is a normally conducting ring of magnets for the electrons. You can even see what's underneath the pink metal cover, but it's not very exciting. Then there's a spin rotator which alters the polarisation of the electrons. A bit further down, you can see the electron and proton rings being brought closer together (the electron beam pipe is the thin bronze-coloured thing just in front of the yellow thing), and then going through the final focusing magnets before colliding with one another in the next room. Not that you can see anything except concrete blocks, because that bit is just way too cool.

And it needs a whole lot of cryogenic stuff to make it work.

PETRA was previously used for particle physics, before being turned into a pre-accelerator for HERA and more recently (last year or so) into a synchrotron radiation source for (e.g.) protein crystallography. This thing is still used - in fact it's one of the most modern synchrotron X-ray sources in the world - but it wasn't switched on while we were in the tunnel, otherwise we would have been fried. Naturally it's hidden behind a huge wall of concrete blocks.

There are plenty more photos to see beyond the ones linked here..!