Package: src:glibc Version: 2.30-4 Severity: wishlist X-Debbugs-CC: address@hidden The ARMv8.1 spec, as implemented by the ARM Neoverse N1 processor, introduces a set of instructions [1] that result in significant performance improvements for multithreaded applications. Sample code demonstrating the performance improvements is attached. When run on a 16-core Neoverse N1 host with glibc 2.30-4, runtimes vary significantly, ranging from lows around 250ms to highs around 15 seconds. When linked against glibc rebuilt with support for these instructions, runtimes are consistently <50ms. Significant performance impact has also been observed in less contrived cases (MariaDB and Postgres), but I don't have a repro to share. Gcc provides two ways to enable support for these instructions at build time. The simplest, and least disruptive, is to enable -moutline-atomics globally in the arm64 glibc build. As described at [2], this option enables runtime checks for the availability of the atomic instructions. If found, they are used, otherwise ARMv8.0 compatible code is used. The drawback of this option is that the check happens at runtime, thus introducing some overhead on all arm64 installations. The second option is to provide libraries built with explicit support for the ARM v8.1a spec via the -march=armv8.1-a flag. This option is also described at [2]. This build would be incompatible with earlier versions of the spec, so it would need to be provided in a location where the linker will automatically discover it if it is usable (e.g. /lib/aarch64-linux-gnu/atomics/). This does not incur any runtime overhead, but obviously involves an additional libc build, and the corresponding complixity and disk space utilization. I'm not sure if this is an option that the glibc maintainers are interested in pursuing. I've tested both options and found them to be acceptable on v8.1a (Neoverse N1) and v8a (Cortex A72) CPUs. I can provide bulk test run data of the various different configuration permutations if you'd like to see additional data. I can provide patches or merge requests implementing either option, at least for a starting point, if you'd like to see them. Thanks! noah 1. https://static.docs.arm.com/ddi0557/a/DDI0557A_b_armv8_1_supplement.pdf Section B1 2. https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html
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