Rusty's Bleeding Edge Page

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So two weeks back was the Official Handing Over Of The SAMBA Team T-shirt! Since then I have done my first serious push to the git tree, and received spam from the build farm about it (false positives, AFAICT).

I'm still maintinging virtio and the module and parameter code of course. But the kernel has slowly morphed into a complicated and hairy place. Formality has crept in, and the pile of prerequisites grows higher (eg. git, checkpatch.pl, Signed-off-by). This is maturity, but it raises the question: when will some neat lean OS without all this baggage come along?

SMP, micro-optimizations, multithreading and extreme portability are responsible for much of the added coding burdens, but also hyper-distributed development means many coders shy away from changes which would break APIs. The suboptimality accretes and this method of working becomes the new norm. BUG_ON() for API misuse is now seen as unduly harsh, but undefined APIs make the next change harder, and WARN_ON() tends to stay around forever.

SAMBA has some brilliant ideas which coding a joy (talloc chief among them, but there are other gems to be found). Hell, it even has a testsuite! But of course it has its own issues; the SAMBA 3/4 split, lack of the kernel's massive human resources and the inevitable code quality issues. Ask me again in a few years to do a comparison...

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I mentioned in my previous post that we had seen tdb corruption (despite the carefully written syncing transaction code) when power failures occurred.

I mentioned (from my previous experience with trying to test virtio_blk) that ext3 doesn't use barriers by default, and that the filesystems should be mounted with "barrier=1". (The IBM engineers on the call were horrified that this wasn't the default: I remember the exact same feeling when I found out!).

I had my tdb_check() routine now, so I patched it into tdbtool and modified tdbtorture to take a -t ("do everything inside transactions") option: killing the box should still allow tdb_check() to pass when it came back. I thought using virtualization, but this isn't easy: killing kvm still causes outstanding writes to be completed by the host kernel (nested virtualization would work). So instead, it was time to use my physical test box.

First with standard ext3. Three times I started tdbtorture -t, then pulled the cord out the back. The first two times, sure enough, the tdb was corrupt. The third time, the root filesystem mounted read only and I fscked, rebooted, same thing, fscked again, rebooted happy. Sure enough, the tdb was corrupt (and one of my previous saved corrupted tdbs was lost, another was in lost+found). I should have forced a fsck on every reboot.

So I edited /etc/fstab to put barrier=1 in, and pulled the plug during tdbtorture again. Surprisingly, I got a journal error and r/o remount again, which shouldn't happen. Still, when I did another double-fsck, the tdb was clean! Two more times (no more fs corruption), and two more clean tdbs.

So it seems, lack of barriers was the culprit. But also note that tdbtorture was 4.8 seconds without barriers, 20 or 28 seconds with them (and this slowdown itself might make errors less likely). This is worse than the 10% that googling suggested, but then tdbtorture is pretty perverse. Three processes all doing three fsyncs per commit, and a commit happening about every 10 db operations.

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A few years back at a netconf, someone (Robert Olsson maybe? Jamal Salim?) got excited about Bloom Filters. It was my first exposure.

The idea is simple: imagine a zeroed bit array. To put a value in the filter you hash it to some bit, and set that bit. Later on, to check if something is in the filter, you hash it and check that bit. Of course this is a pretty poor filter: it never gives false negatives, but has at about {num entries} in {num bits} chance of giving false positives. The trick is to use more than one hash, and the chances of all those bits being set drops rapidly.

It can be used to accelerate lookups, but we never found a good use for it. Still, it sat in the back of my head for a few years until I came across a completely different use for the same idea.

TDB (the Trivial DataBase) is a simple key/value pair database in a file (think Berkley DB). It has a free list head and set of hash chain heads at the start, and each record is single-threaded (via a "next" entry) on one of these lists. My problem is that even though TDB supports transactions, there were reports of corruption on power failure (see next post!); we wanted a fast consistency check of the database. In particular, this was for ctdb: if the db is corrupt you just delete it and get a complete copy from the other nodes.

A single linear scan would be fastest, rather than seeking around the file. Checking each record is easy, but how do we check that it's in the right hash chain (or the free list), and that each record only appears once? The particular corrupt tdb I was given contained such an infinite loop, which is a nasty failure mode. The obvious thing to do is to seek through and record all the next pointers, and the actual record offsets, then sort the next pointers and see that the two lists match. But that involves a sort and would take 8 bytes per record (TDB is 32 bit, so that's 4 bytes for the next pointer and 4 bytes to remember the actual record offset).

How would we do this in fixed space, even though we don't know how many records there are? What if, instead, we allocate two Bloom filters for each hash chain (and one for the free list)? We put next pointers in the first Bloom filter, and actual located records in the second. At the end, the two should match!

But we can do better than this. Say we use 8 hashes, and 256 bits of bitmap. First off, if the 8 hashes of a value overlap already-set bits, it has no effect and we won't be able to tell if it's missing from the other filter. And if seven bits overlap others (so it only sets one unique bit) then we can't detect a "bad" value which sets that same bit and no other unique bits.

So instead of setting bits, we can flip bits in the bitmap. This means that we can detect a single extra value in one list unless it happens to cancel out its own bits (ie. the hash values all happen to form pairs), and if two values are different they'd need to hit precisely the same bits. This is astronomically unlikely (it's a bit more than 1 in 256! / (8! * 248!), but its still a very small number).

The best bit, of course is that you don't need two bitmaps: a single one will do. Since the two sets of values should be equal, it should be all zero bits when finished!

In practice, all the corrupt TDBs I've gathered have had much more gross errors. But it's nice to finally use Bloom's ideas! The code can be found in the CCAN repository.

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It's been a while, but I find myself hacking to 3am tonight (virtio_blk needed some love, and it was easier to patch it myself than explain The Right Thing to the patch submitter).

Am seriously tempted to do that tdb hacking now, but I get Arabella tomorrow and I definitely want to face her fully refreshed!

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So, New York Times covered Sandia National Labs's using a million virtual machines to research botnets. I saw something fly by on slashdot, but didn't pay any attention.

Then a couple of IBM research guys sent me an embarrassed mail a few days ago. Senior IBM execs had seen the Sandia press release crediting lguest which "was developed by the research arm of IBM" and were wondering why they'd never heard of it. :)[1]

Upshot is: I always said lguest was a hypervisor research and education tool, but this blew me away! Ron Minnich has been submitting stuff for lguest for a while now, but I assumed he was just idling. Great stuff!

[1] Linux Technology Center is not part of IBM Research, and lguest was just a random hack I did to help some other things I was working on (and probably spent too much time on to justify).
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Finally submitted to LCA 2010. Yes, I'm submitting the lguest tutorial for the third (and last) time; having put all the effort into it, I feel it will finally be a good tutorial.

But I wanted to talk about something else, so I made a more off-the-wall submission, on the stuff I've been doing with Arabella, the Nintendo Wiimote, and libcwiid. The hope is that if that get accepted it'll give me motivation to spend more time perfecting it!

In unrelated news, I got an email from Peter Richards who has been playing with my old pong code, and made improved IR pens. He had some Vishay IR LEDs left over, and has mailed them to me. If my paper is accepted, I'll have to figure out what to do with them!

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