Nearly one year ago, on November 15th 2015, I released the 1.0 version of nom, the fast parser combinators library I wrote in Rust. A lot happened around that project, and I have been really happy to interact with nom users around the world.
What does profile-guided optimization mean?
Some languages have a JIT (Just In Time) compiler available at runtime, that can optimize the executed code depending on current execution patterns. This is, in large part, the cause of the performance of Lua and the JVM. They can start a bit slow, but by accumulating information on actual running code, they make it faster and faster for the current load. PfLua is a great example: the firewall rules are optimized again and again, until the current network traffic is handled as quickly as possible.
When you use other languages, such as C, you usually cannot optimize the application once it is compiled. Except when you use an optimization technique known as Profile-Guided Optimization. From Wikipedia :
Profile-guided optimization (PGO, sometimes pronounced as pogo), also known as profile-directed feedback (PDF), is a compiler optimization technique in computer programming that uses profiling to improve program runtime performance.
It relies on profiling the compiled application, while it runs with the expected, real world load (web traffic, calculations, etc), and feed this profiling information to the compiler. On the next build, the compiler will have more information on which parts of the program are less used, which branches are taken more often, the expected values in a range, etc. Instead of guessing how the program would behave to choose optimizations, the compiler has true information, and can optimize more precisely. There’s one issue with the process: you need two compilations and a profiling run to generate the final executable. But it gets easier when you automate it, as we can see in the Firefox build process.
PGO in LLVM
While it has been available in other systems for a long time (Visual Studio 2005, the Intel compiler ICC for Itanium), it appeared recently in LLVM. It has since then been applied successfully to XCode (Objective C, Swift) and LDC, the D compiler.
LLVM has a great feature: it uses an Intermediate Representation code (IR), which is a kind of high level assembly language. It applies its optimizations and machine code generation to that representation. If you make a compiler for a new language, targeting the LLVM IR will give you these features (nearly) for free.
In practice, compiler frontends choose which features they use, so you may not access everything LLVM has to offer. In particular, the Rust compiler, as of now (April 2016), provides a llvm-args option, but that option filters what you can send to LLVM, so we cannot use PGO here.
PGO in Rust
Still, with rustc, you can generate directly the IR, or its binary encoding, named bitcode:
rustc –emit llvm-bc main.rs
# or, with cargo:
cargo rustc — –emit llvm-bc
The approach I tried here is to take that bitcode, and manually apply LLVM’s transformations until I get a compiled executable. This is not really usable for now, especially because I chose an example with very few dependencies. With more dependencies, the compilation and linking will get more complex and unmanageable manually.
LLVM comes with a few commands that you can use to build code manually. The first one is opt, and it applies optimizations and instrumentation on the bitcode file (here, the file target/release/pgo.bc):
opt-3.8 -O2 -pgo-instr-gen -instrprof target/release/pgo.bc -o pgo.bc
The new bitcode file contains code to profile the end application (mainly by counting how often we use each code path). We can now convert that bitcode file to an object file, and link it using clang:
llc-3.8 -O2 -filetype=obj pgo.bc
clang-3.8 -O2 -flto -fprofile-instr-generate pgo.o -L/usr/local/lib/rustlib/x86_64-apple-darwin/lib -lstd-ca1c970e -o pgo
Note: I built my own rustc from source, so your libstd file may not have the same hash. Since Rust (as of April 2016) uses LLVM 3.7, we can use LLVM 3.8’s PGO features, since the bitcode format is apparently backward compatible. I use OS X, and Homebrew’s LLVM 3.8 has compilation issues, so I needed to build the compiler runtime from source. It’s a proof of concept, not production code 😉
We will now run the program we just built, preferably with production data and traffic. It will automatically generate a default.profraw file, containing the profiling information. This file must be transformed to a format that opt will understand with llvm-profdata:
llvm-profdata-3.8 merge -output=pgo.profdata default.profraw
This .profdata file will now be used in the compilation steps:
opt-3.8 -O2 -pgo-instr-use -pgo-test-profile-file=pgo.profdata target/release/pgo.bc -o pgo-opt.bc
llc-3.8 -O2 -filetype=obj pgo-opt.bc
clang-3.8 -O2 -flto -fprofile-instr-use=pgo.profdata pgo-opt.o -L/usr/local/lib/rustlib/x86_64-apple-darwin/lib -lstd-ca1c970e -o pgo-opt
We now have an executable compiled using profiling information. Is it fast?
The program I tested is a n-body simulation. It was a great test target since libstd is the only dependency, and the load factor depends on a number given as command line argument. Here is a test with time (I know it’s not the most precise benchmarking tool, but for a tenth of second precision, it works alright):
$ time ./target/release/pgo 1000000000
$ time ./pgo-opt 1000000000
As it turns out, we gain nearly 15% in running time on this program. Other examples could have less impact, but this is encouraging! So, what happened inside our program?
The generated code
I provide assembly dumps of the normal program, generated with cargo –release, and the one optimized with PGO. Mostly, the code has been reordered, probably to fit better in cache lines. You can also consult PDF files with call graphs: normal, PGO optimized.
The whole code for this article is available here if you want to reproduce the results or tinker with optimizations yourself.
This is a proof of concept, demonstrating that profile guided optimization could work in Rust. It is probably worthy of integration into rustc, but there’s a lot of work before it could be usable. Still, there’s a github issue where you can weigh in, if you would like this optimization in your applications.
I’m getting less and less satisfied with Twitter to exchange thoughts. The 140 characters is not the obvious problem, since you can chain messages easily. The issue is that those thoughts are ephemeral. This medium does not optimize for smart discussion with relevant people, but for quick wit from currently available people, before being dumped under a stack of comments on the latest news. The retweeting does not help much, since the primary reason for retweeting are 1. it’s funny 2. it is shocking 3. it’s inspiring, and long last “maybe it’s interesting”. They don’t create much discussion.
Until now, I have primarily used this blog for long posts (thus explaining why I don’t write much here). As my friends say “if it’s more than 3 tweets, write a blog post”.
So in the following months, I’ll try to post short, not well researched but spontaneous articles, instead of ranting in 140 characters.
When networks began to expand and people saw the need for secure communication, they designed complex systems based on public key cryptography, that worked more or less. Problem: how do you trust that the key a server sent you is the right one? How can you make sure that it is not somebody else trying to impersonate that website?
Multiple solutions were proposed, and the most promising was a public directory of domain names and associated public keys, maintained by a peer to peer network named KeyCoin. It looked better than so called Web Of Trust solutions, because everybody could agree on what was the correct key for a given domain. As long as nobody hold 51% of the network, no change could happen without being validated by a lot of different peers. The network was maintained by 10000 enthusiast system administrators who took their task very seriously (after all, the security of the whole system depended on their honesty), and nobody had enough computing power to take over the network.
After a while, people began using the system, since it was directly integrated in their browsers, but they did not want to run a node on the network themselves. It was too bothersome, and they could trust the administrators. Also, they had to ask one of them to make a change everytime. The whole process was a bit artisanal.
In the meantime, some people demonstrated the 51% attack on networks of reduced size, and that worried people. They wanted a safe system, one that was not only relying on those sysadmins that could do anything. Who were they anyway? Running that system was still too complex for non technical too run it themselves anyway, so they did not worry enough. But some governments found that rewriting the truth of name/key matching was interesting. Maybe to catch pedophiles, terrorists, criminals. Or maybe to censor websites, I do not know, they told me it was for my own good.
Some smart person found a good solution: if controlling the whole system necessitated owning 51% of the system, the easiest way was to have a lot of machines, enough to counteract the sysadmins. That did not seem risky when people designed the system. Nobody could have enough computing power to take over the whole network, and there would be even more nodes every day.
Yet, that person got enough funding to install tens of thousands of machines and make them join the network. They even provided a nice enough interface for people and businesses to input their domain name and public key, as long as they paid some fees. The sysadmins welcomed him at first, since money coming in the system validated their ideas. Atfer a while, they started worrying, since none of them could keep up with the computing power, but that company asssured them it would never attain 51% of the network.
Other companies jumped on the bandwagon and started to profit from that new business opportunity. Governments started their own server farms to participate too. Problem: now that everybody (except the sysadmins) had a lot of computing power, nobody had enough to control the network entirely.
So they started making alliances. If a few major players work as a team, they can do whatever they want on the network. If one of them decided to try and replace a key on the ledger, others could help it. Of course, once they begun doing that, others wanted to participate. So they created a few rules to join their club. First, you needed to have enough machines. That was a good rule, because that made a big barrier to entry. You could not start as a small player. The other rules? You had to submit to an audit, performed by the other players. Yet another barrier to entry. And once they deemed you acceptable, you had to follow the requests of governments, which were arbitrarily refusing candidates.
Even with the big barriers to entry, a few hundred players came up, often backed by governments. Of course, all ended up in the same team, doing whatever they wanted, as long as nobody was complaining, because anytime one of them had something shady to do, all of them followed automatically.
Since building those big companies required money, they made their clients pay more and more, and to make it easier to accept, provided “premium” options where they show they trust you more, since they took the time to phone your company and ask a few questions.
Some found that big system too centralized, too obedient to states, and decided to fork it. There are separate public ledgers, but they do not come directly embedded in browsers, you need to integrate them yourself, and that’s bothersome. Also, most of those networks have a few hundred nodes at best.
From a nice, decentralized, home made system, we ended up with a centralized system controlled by corporations and governments.
Now let me tell you about that system I designed. It is based on a concept named certificate, a cryptographically signed file that links the public key to a domain name. Now here’s the catch: a certificate represents a key, and is signed by another key, which is represented by another certificate, and so on and so forth until a certificate that signs itself. That system is good, because you just have to embed the root certificate that your friends gives you, and you’ll be able to verify the key of his websites, even if those keys change. And this, without even asking the public ledger, so that is a truly decentralized and more anonymous system! Nothing could go wrong with that, right?
I do not know who started this argument a few days ago. It feels like something coming from HN. Do you need to know mathematics to be a good programmer?
There is a lot of differing opinions. Maybe programming is a subbranch of mathematics, or programming is using mathematics. Or learning programming is closer to learning a new language. For me, saying that programming is about languages is like saying that literature is about languages. Sure, you need words to indicate concepts, some languages are better suited than others for that, and some concepts are better expressed in other languages. It is more like a hierarchy to me: philosophy formalizes concepts used by authors to write in common languages. Mathematics formalize concepts used by programmers to create code in common languages.
But this is besides the point.
This debate sparks outrage, since it touches a central point of our education, and one that is often not taught very well. “Look, I do not use geometry while writing a loop, so maths are pointless for me”. A lot of developers will never learn basic algebra or logic and will never need it in their day job. And that’s okay.
Programming is not a single profession anymore. Each and every one of us has a different definition. A mechanical engineer working on bridges, another on metallic parts for cars and another one on plastic toys all have different needs, different techniques for their job, although the fundamental basis (evaluating breaking strength, time of assembly, production costs) is the same. That does not make one of these jobs worth more than the other.
The real problem is that we are still fighting among ourselves to define what our job is. The other pointless debate, about software being engineering, science or craft, is evidence of that. And it will stay hard to define for a long time.
We are in a unique position. Usually, when a new field emerges, either tinkerers are launching it and later, good practices are studied to make it engineering, or scientists create it, then means of production become cheaper and crafters take over.
Computers were started by scientists, but the ease of access gave crafters a good opportunity to take over. But that does not mean research stopped when people started coding at home. So now, in a relatively new field (less than a century), while we are still exploring, we have a very large spectrum of jobs and approaches, from the most scientific to the most artistic kind. And that is okay. More world views will help us get better at our respective jobs.
So, while you are arguing that the other side is misguided, irrealistic or unrigorous, take time to consider where they come from. They do not have the same job, and that job can seem pointless to you, but they can be good at it, so there is probably something good you can learn from their approach. The only thing you should not forgive from the other side is the lack of curiosity.
Since WhatsApp announced its acquisition, a lot of people started to switch to alternatives, trying to escape from Facebook. Some of them then discovered my article about Telegram, and a common answer was “hey, at least, it is better than WhatsApp, because it is open source, faster and it has encryption”.
This is a very bad way to decide what application you should use. If you choose a secure messaging app, it must be because you need it, not just because you want to avoid Facebook.
Those are not good enough requirements:
- independent from Facebook
- multi platforms
- open source
Yes, even open source, because it does not magically make software safe.
So, what are goods requirements? Well, I already have a list of what a secure messaging app should meet to be considered. If an app does not follow those requirements, it may not be a good idea to use it.
But it still does not mean the app will fit your use case. So you must define your use case:
- Why do you need it?
- With whom will you communicate?
- Who is the adversary?
- What will happen if some of your information is revealed to the adversary?
- Does it need to be always available?
- For how long will it be used?
This is part of what I mean when I insist on having a threat model: you cannot choose correctly if you do not know the risks.
Here are a few examples that you could consider.
The activist in a protest
The activist must be able to communicate quickly in the crowd. Identifying info might not be the most important part, because she can use burner phones (phones that will be abandoned after the protest). The most important feature is that it should be always available. Phone networks were often used to disrupt activist communication, so a way to send message through WiFi our bluetooth might be useful. The messages can be sent to a lot of different people, so being able to identify them might be important. If it is large enough to be infiltrated easily, then having no way to identify people is crucial.
Being able to send photos is important, because they might be the only proof of what happened in the protest. Here, I have in mind the excellent ObscuraCam app, which is able to quickly hide the faces of people in photos before sending them.
The application should not keep logs, or provide a way to quickly delete them, or encrypt them by default, because once someone is caught, the police will look through the phone.
The crypto algorithms and protocols should be safe and proven for that use case, because the adversaries will have the resources to exploit any flaw.
No need for a good update system if the devices will be destroyed after use.
The employee of a company with confidential projects
The adversaries here are other companies, or even other countries. The most important practice here is the “need to know”: reduce the number of persons knowing the confidential information. that means the persons communicating between themselves is reduced, and you can expect that they have a mean of exchanging information securely (example: to verify a public key).
Identifying who talks with whom is not really dangerous, because it is easy to track the different groups in a company. You may be confident enough that the reduced group will not be infiltrated by the adversary. The messages should be stored, and ideally be searchable. File exchange should be present.
There could be some kind of escrow system, to reveal information if you have a certain access level. Authentication is a crucial point.
The crypto may be funnier for that case, because the flexibility needed can be provided by some systems, like identity based encryption.Enterprise policies might be able to force regular uodates of the system, so that everybody has the same protocol version at the ame time, and any eventual flaw will be patched quickly.
The common user
It is you, me, anyone wanting to exchange private messages with friends or family. Here, trying to protect against the NSA is futile, because most of the contacts might not have the training needed. Trying to hide the contacts list from Facebook is futile too: even if someone protects the information, one of the contacts may not. The adversary you should consider here: crooks, pirates, anyone that could exploit the private messages for criminal ways (stealing bank info, blakcmailing, sending malware, etc).
An application fitting this use case should encrypt messages, preferably end to end, to limit problems when the exchange server is compromised. The service might not provide any expectation of anonymity. Messages should be stored, but encrypting them is a good option, in case the device is lost or stolen.
The crypto does not need to be very advanced, but it should use common, well known designs.
There should be a good update system, a way to negotiate protocol versions (and forbid some unsafe versions), because you will never be sure that everybody has performed all the needed updates.
Your use case here
Those were some common situations, for which some solutions exist, but there are a lot more possible use cases. If you are not sure about yours and need help defining your threat model, do not hesitate to ask for help, and do not jump on a solution because the marketing material says it is safe.
A good security solution will not only tell you what is protected, and how, but also what is not protected, and the security margins you have. It will also teach you the discipline you need to apply to get the most out of it.
Everytime people discuss the hacker community and its diversity, I see someone waving the “meritocracy” argument. “It is not our fault those minorities are not well represented, if they knew more stuff or did more stuff, they would have a better status”.
It is easy to see how that argument would be flawed, as meritocracy is a power structure, and whenever a power structure is created, after some time it tends to reinforce its own community. But that is not my point right now.
I realized that the idea of meritocracy is so deeply ingrained in the hacker mindset that we lost sight of what was important. I can see how that idea is appealing. Once you prove you know stuff, people will recognize you, and that will be enough to motivate you to learn. Except it is not. The meritocracy is just another way to exclude people. Once you consider someone’s status by how much you perceive they know, things go downhill.
Some are good at faking knowledge. Some know their craft, but do not talk that well. Some are not experts, but have good ideas. Some would like to learn without being judged. Everytime you dismiss someone’s opinion because of their apparent (lack of) knowledge, everytime you favor someone’s opinion because of their apparent knowledge, you are being unscientific and unwelcoming. You are not a hacker, you are just a jerk.
Somewhere along the way, people got too hung up on meritocracy, and forgot that you hack for knowledge and for fun, not for status. It is all about testing stuff, learning, sharing what you learned, discussing ideas and helping others do the same, whatever their skills or their experience. Status and power structures should have nothing to do with that.
Guess what? I pointed out that bad behaviour, but I am guilty of it too. I have to constantly keep myself in check, to avoid judging people instead of judging ideas. That’s alright. Doing the right thing always requires some effort.