Hi All Consider this a tentative first email to test the water, but I have started to look at performance of particularly the install phase of the emerge utility and I could use some guidance on where to go next
Firstly, to define the "problem": I have found gentoo to be a great base for building custom distributions and I use it to build a small embedded distro which runs on a couple of different architectures. (Essentially just a "ROOT=/something emerge $some_packages"). However, I use some packaging around binpackages to avoid uncessary rebuilds, and this highlights that "building" a complete install using only binary packages rarely gets over a load of 1. Can we do better than this? Seems to be highly serialised on the install phase of copying the files to the disk? (Note I use parallel build and parallel-install flags, plus --jobs=N. If there is code to compile then load will shoot up, but simply installing binpackages struggles to get the load over about 0.7-1.1, so presumably single threaded in all parts?) Now, this is particularly noticeable where I cheated to build my arm install and just used qemu user-mode on an amd64 host (rather than using cross-compile). Here it's very noticeable that the install/merge phase of the build is consuming much/most of the install time. eg, random example (under qemu user mode) # time ROOT=/tmp/timetest emerge -1k --nodeps openssl >>> Emerging binary (1 of 1) dev-libs/openssl-1.1.1k-r1::gentoo for >>> /tmp/timetest/ ... real 0m30.145s user 0m29.066s sys 0m1.685s Running the same on the native host is about 5-6sec, (and I find this ratio fairly consistent for qemu usermode, about 5-6x slower than native) If I pick another package with fewer files, then I will see this 5-6 secs drop, suggesting (without offering proof) that the bulk of the time here is some "per file" processing. Note this machine is a 12 core AMD ryzen 3900x with SSDs that bench around the 4GB/s+. So really 5-6 seconds to install a few files is relatively "slow". Random benchmark on this machine might be that I can backup 4.5GB of chroot with tar+zstd in about 4 seconds. So the question is: I assume that further parallelisation of the install phase will be difficult, therefore the low hanging fruit here seems to be the install/merge phase and why there seems to be quite a bit of CPU "per file installed"? Can anyone give me a leg up on how I could benchmark this further and look for the hotspot? Perhaps someone understand the architecture of this point more intimately and could point at whether there are opportunities to do some of the processing on mass, rather than per file? I'm not really a python guru, but interested to poke further to see where the time is going. Many thanks Ed W
