Category Archives: dragonflybsd

cache line aliasing effects, or "why is freebsd slower than linux?"

There was some threads on FreeBSD/DragonflyBSD mailing lists a few years ago (2012?) which talked about some math benchmarks being much slower on FreeBSD/DragonflyBSD versus Linux.

When the same benchmark is run on FreeBSD/DragonflyBSD using the Linux layer (ie, a linux binary compiled for linux, but run on BSD) it gives the same or better behaviour.

Some digging was done, and it turned out it was due to memory allocation patterns and memory layout. The jemalloc library allocates large chunks at page aligned boundaries, whereas the allocator in glibc under Linux does not.

I've put the code online in the hope that others can test and verify this:

The branch 'local/freebsd' has my local change to allow the allocator offset to be specified. The offset compounds on each allocation - so with an 'n' byte offset, the first allocation is 0 bytes offset from the page boundary, the next is 'n' bytes offset from the page boundary, the next is '2n' bytes offset, etc.

You can experiment with different values and get completely different behavioural results. It's non-trivial: there's a 100% speedup by using a 127 byte offset for each allocation, versus a 0 byte offset.

I'd like to investigate cache line aliasing effects further. There was work done a few years ago to offset mbuf headers in the FreeBSD kernel so they weren't all page-aligned or 256/512/1024 byte aligned - and apparently this gave a significant performance improvement. But it wasn't folded into FreeBSD. What I'd like to do is come up with some better strategies / profiling guides for identifying when this is actually happening so the underlying objects being accessed can be adjusted.

So - if anyone out there has any tips, hints or suggestions on how to do this, please let me know. I'd like to document and automate this testing.

New release: ELF Toolchain v0.6.1

I am pleased to announce the availability of version 0.6.1 of the software being developed by the ElfToolChain project.

This new release supports additional operating systems (DragonFly BSD, Minix and OpenBSD), in addition to many bug fixes and documentation improvements.

This release also marks the start of a new "stable" branch, for the convenience of downstream projects interested in using our code.

Comments welcome.