On Wed, Feb 28, 2018 at 9:04 PM, <frowand.list@xxxxxxxxx> wrote: > Create a cache of the nodes that contain a phandle property. Use this > cache to find the node for a given phandle value instead of scanning > the devicetree to find the node. If the phandle value is not found > in the cache, of_find_node_by_phandle() will fall back to the tree > scan algorithm. > > The cache is initialized in of_core_init(). > > The cache is freed via a late_initcall_sync() if modules are not > enabled. > > If the devicetree is created by the dtc compiler, with all phandle > property values auto generated, then the size required by the cache > could be 4 * (1 + number of phandles) bytes. This results in an O(1) > node lookup cost for a given phandle value. Due to a concern that the > phandle property values might not be consistent with what is generated > by the dtc compiler, a mask has been added to the cache lookup algorithm. > To maintain the O(1) node lookup cost, the size of the cache has been > increased by rounding the number of entries up to the next power of > two. > > The overhead of finding the devicetree node containing a given phandle > value has been noted by several people in the recent past, in some cases > with a patch to add a hashed index of devicetree nodes, based on the > phandle value of the node. One concern with this approach is the extra > space added to each node. This patch takes advantage of the phandle > property values auto generated by the dtc compiler, which begin with > one and monotonically increase by one, resulting in a range of 1..n > for n phandle values. This implementation should also provide a good > reduction of overhead for any range of phandle values that are mostly > in a monotonic range. > > Performance measurements by Chintan Pandya <cpandya@xxxxxxxxxxxxxx> > of several implementations of patches that are similar to this one > suggest an expected reduction of boot time by ~400ms for his test > system. If the cache size was decreased to 64 entries, the boot > time was reduced by ~340 ms. The measurements were on a 4.9.73 kernel > for arch/arm64/boot/dts/qcom/sda670-mtp.dts, contains 2371 nodes and > 814 phandle values. The question is why O(1) is so important? O(log(n)) wouldn't work? Using radix_tree() I suppose allows to dynamically extend or shrink the cache which would work with DT overlays. -- With Best Regards, Andy Shevchenko -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
