Category Archives: Computers

Computer related articles

Diagnosing faulty memory in Linux…

For the past year I’ve had very occasional chrome crashes (segfaults in rendering process) and an occasional bit of btrfs corruption. As it was always easily repairable with btrfs check --repair I never thought much about it, although I suspected it may be an issue with the memory. I ran memtest86 overnight one time but it didn’t show up any issues. There were never any read or SMART issues logged on the disk either, and it happened to another disk within the machine as well.

Recently though I was seeing btrfs corruption on a weekly basis, especially after upgrading to ubuntu 18.04 (from ubuntu 16.04). I thought it may be a kernel issue so I got one of the latest kernels. It seemed to happen especially when I was doing something quite file-system intense, for example browsing some cache-heavy pages while running a vm with a long build process going on.

Then, earlier in the week the hard drive got corrupted again, much more seriously and after spending some time fixing, running btrfs check --repair a few times it suddenly started deleting a load of inodes. Force rebooting the machine I discovered that the disk was un-mountable, although later I was able to recover quite a lot of key data from btrfs restore as documented in this post.

memtest86 was still not showing any issues, and so my first thought was that assuming the hard disk was not at fault it may be something to do only when the memory had a lot of contention (memtest86 was only able to run on a single core on my box). I booted a minimal version of linux and ran a multi-process test over a large amount (not all) of the memory:

where 8 is the number of processor/threads and 1400 is the amount of free memory on the system divided by that number (in my case I was testing 16gb of memory). 10 is the number of runs. It took about 45 min to run once over the 16gb, or about 25 min to run over 8gb (each of the individual sodimms in my laptop).

Within about 10 minutes it started showing issues on one of the chips. I’ve done a bit of research since this and seen that if a memory chip is going to fail then it would usually do it within the first 6 months of being used. However this is a kingston chip that has been in my laptop since I bought it 2 or 3 years back. I added another 8gb samsung chip a year ago and it seemed to be after that that the issues started, however that chip works out as fine. Perhaps adding another chip in broke something, or perhaps it just wore out or overheated somehow…

Automounting swap on local SSD’s on Amazon EC2

Many instances on EC2 (AWS) now have local SSD’s attached. The excellent ubuntu 14.04 image boots brilliantly on these and automatically formats and mounts any of the local SSD storage. However when the instance shuts down, reboots or gets migrated these SSD’s go away so you still need to use the persistent EBS storage for most operations.

If you want to enable swap on the box, add the following to /etc/rc.local – it will create a 2gb swap file each boot on the local SSD and mount it:

I’ve not yet figured out what the process is to format/mount these local disks on bootup it may well be easier to add this to them.

Facebook Graph API Page post changes

So about a month back it looks like facebook changed their graph API to prevent posting links to pages using the method we had always used which was simply a post to //feed with my access token with message and link parameters. Posting just a message was working fine still but when I tried to add a link in I was just getting access denied.

After spending an hour or two bashing my head against the wall I discovered that you had to first access a list of all your pages with your user access token, then from that you would figure out the page’s special access token, and only then could you post.

So the resulting (somewhat messy) perl code is like:

Extracting old weird format audio files

So, I had a friend who has a load of recordings from about 10 years ago which were done on a weird dictophone. The files had the extension .FC4 which according to the internet is a legacy Amiga audio format with no more support. Great.

First thing was to run file on it:

Great. Let’s see if we can do a better job looking at a hex dump (with xxd):

So, looks like at offset 0x100 (256) we have something that is a RIFF/WAV file, then the stuff that shows as U is probably a chunk-size block or somesuch. Given the blocks of data afterwards it could probably be 16-bit single channel at a guess. Perhaps something can read that if we cut the initial header off and re-save:

Ah-ha looks like file has a clue now. Let’s try to play it:

D’oh. Opening as a raw file in audacity shows pretty much white-noise (whereas you’d have expected it to be something vaguely like speach but with blips in every so often if it was any sort of valid PCM or wave type encoding).

After searching around for a long time I discovered this post which talked about a very similar looking header and especially WAV encoding 0x350. This linked to an mplayer plugin with an acm and inf file however the ubuntu version of mplayer doesn’t support w32codecs. I tried installing this in several different ways in a windows 7 vm but couldn’t get it to work.

I then tried compiling mplayer from source only to be greeted with:

D’oh. Rather than mess around with trying a 32-bit compile or hacking the assembly I remembered I had a 10-year old laptop lying around with a very old 32-bit install of gentoo. Power it up, install the codec files and it plays them!

I then try to extract some proper PCM WAV file from the FC4 file using mencoder. But mencoder doesn’t support audio-only. I also try using the -dumpstream option in mplayer but that just dumps the encoded audio. So finally I come across the -ao pcm option which puts out a nice plain wav file that I can encode into mp3 or any other format.

Extracting images from word documents and turning into PDF

So someone recently sent me a load of Word 2007 documents that had musical scores embedded as pictures. Nice and easy to convert these (losslessly) into PDF’s with one image per page using this img2pdf script