Has there been any thought about perf supporting function reordering? The kernel had a function reorder option that was available in Linux 2.6.17 to 2.6.21? The kernel code has poor performance when compared to the user-space code. For a simple experiment compiling the kernel code the kernel was getting a L1 icache miss every 60 instructions versus the userspace getting an L1 icache miss every 260 instructions. The difference in IPC was also significant. The kernel code had 0.55 IPC while userspace had 1.59 IPC.
The arguments for removal of the function reorder code for Linux 2.6.22 were: -linker was slowed too much by many sections -manual generation of the ordering list got out of date -too diverse worksloads With perf it is easy to collect information about the operation runtime characteristics. Over the weekend I was able to collect call graph information of kernel build on the system with perf, then render the data with gprof2dot and dot: export training=training/make_a_g_branch_k sudo perf record -a -g -e branches:k -o $training.data su wcohen -c "make -j4" sudo chown wcohen $training.data; sudo chgrp wcohen $training.data perf script -i $training.data| gprof2dot --format=perf > $training.gv dot -Tsvg < $training.gv > $(training).svg By default gprof2dot prunes nodes and edges. The following provides a more complete graph: perf script -i $training.data| gprof2dot --format=perf -n 0.05 -e 0.01 > training/make_a_g_branch_k_2.gv The results graphs are at: http://people.redhat.com/wcohen/sediment/make_a_g_branch_k.svg http://people.redhat.com/wcohen/sediment/make_a_g_branch_k_2.svg The graphs give some indication of the flow through the kernel code. Search for "system_call" will show the kernel entry and where thing branch out from there. Other place of interest "page_fault" and "apic_timer_interrupt". Any thoughts on making it easier for perf make this statistical callgraph information available and using it to do code reordering? I have experimented with code reorder with user space postgres package and it did help performance about 5% improvement in IPC (http://people.redhat.com/wcohen/sediment/html/pop.html) -Will -- To unsubscribe from this list: send the line "unsubscribe linux-perf-users" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
