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> We of course also compared gcc with clang/LLVM and found that clang was 0.8% slower, so it does not explain the difference entirely. I think it would be quit

> We of course also compared gcc with clang/LLVM and found that clang was 0.8% slower, so it does not explain the difference entirely. I think it would be quite useful and interesting to figure out why there is still a small difference between clang and Rust. There might be some low hanging fruit in how Rust generates LLVM-IR that could be fixed.

emmericp 7y ago
Another thing to point out: our 6% - 11% difference is already quite low. Netbricks [1] has a similar comparison between a Rust and a C network function (only the NF, driver in C in both cases) and they find 14% (LPM) to 20% (synthetic NF) difference. Fun fact: we've a system where Rust is faster than C despite still using more instructions, so yeah, neither instructions nor cycles tell the whole story... [1] https://people.eecs.berkeley.edu/~apanda/assets/papers/osdi16.pdf https://people.eecs.berkeley.edu/~apanda/assets/papers/osdi1...
- kbenson 7y ago
  > Fun fact: we've a system where Rust is faster than C despite still using more instructions I think Bryan Cantrill did a pretty good explanation on differences you might see when rewriting something in Rust[1], and one of the things he looked at to see what was going on was Cycles Per Instruction. Instruction count itself means little if the instructions themselves have very different performance profiles and require different amounts of cycles to complete. Edit: You are tracking and reporting that, so it's not like I'm telling you anything you don't know. I still think the included article is well worth reading though. 1: http://dtrace.org/blogs/bmc/2018/09/28/the-relative-performance-of-c-and-rust/ http://dtrace.org/blogs/bmc/2018/09/28/the-relative-performa...
  - StreamBright 7y ago
    Thanks for sharing this, I haven't seen this video before but it i mind blowing.
  - StavrosK 7y ago
    Not only that, but your tight loop that takes up most of the execution time might have fewer instructions, whereas the rest of the program might have more, making the overall total higher but "instructions per time spent in function" lower.
- fluffything 7y ago
  Since the resources of that paper are not available anymore, do you happen to know if they used Clang for compiling their C code ? A 10% perf difference between LLVM and GCC for different applications is in the order of magnitude of what all the hundreds of Phoronix benchmarks show every time a new version of these toolchains is released for general applications (e.g. not for micro-kernels).
ynx 7y ago
doesn't it also use an older fork of LLVM rather than branching/rebasing off of master? That could account for losing out on optimizations.
- fluffything 7y ago
  The Rust toolchain distributed by the Rust project bundles an LLVM version that's very close to LLVM master. IIRC, this version has some patches on top to allow Rust to query some backend information, but I'm not sure if this information is still up-to-date. Rust can, however, work with an external LLVM. When installing Rust through a package manager, e.g., on Linux, typical Linux distros like Debian, RH, Ubuntu, etc. configure Rust to use the system-wide LLVM binaries. So if you have Debian, and say LLVM 8.0 installed, you can just compare the installed clang 8.0 with the Rust version installed by the Linux distro. That will not compare the "bleeding edge" clang vs Rust toolchains, but it would be a fair comparison of Clang vs Rust at a particular point in those toolchains lifes.

emmericp 7y ago

Another thing to point out: our 6% - 11% difference is already quite low. Netbricks [1] has a similar comparison between a Rust and a C network function (only the NF, driver in C in both cases) and they find 14% (LPM) to 20% (synthetic NF) difference. Fun fact: we've a system where Rust is faster than C despite still using more instructions, so yeah, neither instructions nor cycles tell the whole story... [1] https://people.eecs.berkeley.edu/~apanda/assets/papers/osdi16.pdf https://people.eecs.berkeley.edu/~apanda/assets/papers/osdi1...

kbenson 7y ago
> Fun fact: we've a system where Rust is faster than C despite still using more instructions I think Bryan Cantrill did a pretty good explanation on differences you might see when rewriting something in Rust[1], and one of the things he looked at to see what was going on was Cycles Per Instruction. Instruction count itself means little if the instructions themselves have very different performance profiles and require different amounts of cycles to complete. Edit: You are tracking and reporting that, so it's not like I'm telling you anything you don't know. I still think the included article is well worth reading though. 1: http://dtrace.org/blogs/bmc/2018/09/28/the-relative-performance-of-c-and-rust/ http://dtrace.org/blogs/bmc/2018/09/28/the-relative-performa...
- StreamBright 7y ago
  Thanks for sharing this, I haven't seen this video before but it i mind blowing.
- StavrosK 7y ago
  Not only that, but your tight loop that takes up most of the execution time might have fewer instructions, whereas the rest of the program might have more, making the overall total higher but "instructions per time spent in function" lower.
fluffything 7y ago
Since the resources of that paper are not available anymore, do you happen to know if they used Clang for compiling their C code ? A 10% perf difference between LLVM and GCC for different applications is in the order of magnitude of what all the hundreds of Phoronix benchmarks show every time a new version of these toolchains is released for general applications (e.g. not for micro-kernels).

ynx 7y ago

doesn't it also use an older fork of LLVM rather than branching/rebasing off of master? That could account for losing out on optimizations.

fluffything 7y ago
The Rust toolchain distributed by the Rust project bundles an LLVM version that's very close to LLVM master. IIRC, this version has some patches on top to allow Rust to query some backend information, but I'm not sure if this information is still up-to-date. Rust can, however, work with an external LLVM. When installing Rust through a package manager, e.g., on Linux, typical Linux distros like Debian, RH, Ubuntu, etc. configure Rust to use the system-wide LLVM binaries. So if you have Debian, and say LLVM 8.0 installed, you can just compare the installed clang 8.0 with the Rust version installed by the Linux distro. That will not compare the "bleeding edge" clang vs Rust toolchains, but it would be a fair comparison of Clang vs Rust at a particular point in those toolchains lifes.