> On 9 Jan 2019, at 09:53, Raj <rajlistu...@gmail.com> wrote: > > Hello all, > > While going through the VPP code, I have not fully understood what > the following functions mean and when should they be used? > > 1. static_always_inline void > vlib_buffer_enqueue_to_next (vlib_main_t * vm, vlib_node_runtime_t * node, > u32 * buffers, u16 * nexts, uword count) > > I could see following comments about this function > /* > * Send the packets along their respective next-node graph arcs > * Considerable locality of reference is expected, most if not all > * packets in the inbound vector will traverse the same next-node > * arc > */ > I tried reading through the code but could not figure out what this > function does, and how this is implemented. I see that the code has > optimizations for various CPU, but not could not figure out what those > optimizations are.
This function simply gets array of buffer indices and array of next indices, and enqueues those buffers to be processed by those next nodes... > > 2. #define vlib_prefetch_buffer_header (b, type ) > CLIB_PREFETCH (b, 64, type) > > This gets expanded to: > #define _CLIB_PREFETCH(n,size,type) \ > if ((size) > (n)*CLIB_CACHE_LINE_BYTES) \ > __builtin_prefetch (_addr + (n)*CLIB_CACHE_LINE_BYTES, \ > CLIB_PREFETCH_##type, \ > /* locality */ 3); > > type can be either LOAD or STORE. What is the purpose of these two operations? > > I am sure I am missing some thing with respect to understanding > prefetch. I assume LOAD will prefetch data from memory, and STORE > performs its inverse operation? On x86, there was only load prefetch for a long time, recently store prefetch is introduced with Skylake microarchitecture. You can find explanation here: https://www.felixcloutier.com/x86/prefetchwt1 <https://www.felixcloutier.com/x86/prefetchwt1> Please note that compiler will emit PREFETCHWT1 instruction only if your code is using multi-versioning in VPP (use of VLIB_NODE_FN() macro + MULTIARCH_SOURCES in CMakeLists.txt file). and only when skylake is detected on runtime. Otherwise CLIB_PREFETCH (x, 64, STORE) will produce identical results like you did LOAD.... -- Damjan
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