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One regcache object is created for each stopped thread and is stored in the regcache::regcaches linked list. Looking up a regcache for a given thread is therefore in O(number of threads). Stopping all threads then becomes O((number of threads) ^ 2). It becomes noticeable when debugging thousands of threads, which is typical with GPU targets. This patch replaces the linked list with an std::unordered_multimap, indexed by (target, ptid). I originally designed it using an std::unordered_map with (target, ptid, arch) as the key, because that's how lookups are done (in get_thread_arch_aspace_regcache). However, the registers_changed_ptid function, also somewhat on the hot path (it is used when resuming threads), needs to delete all regcaches associated to a given (target, ptid) tuple. Using (target, ptid) as a key allows to do this more efficiently (see exception below). If the key of the map was (target, ptid, arch), we'd have to walk all items of the map. The lookup (in get_thread_arch_aspace_regcache), walks over all existing regcaches belonging to this (target, ptid), looking to find the one with the right arch. This is ok, as there will be very few regcaches for a given key (typically one). Lookups become faster when the number of threads grows, compared to the linked list. With a small number of threads, it will probably be a bit slower to do a map lookup than to walk a few linked list nodes, but I don't think it will be noticeable in practice. The function registers_changed_ptid deletes all regcaches related to a given (target, ptid). We must now handle the different cases separately: - NULL target and minus_one_ptid: we delete all the entries - NULL target and non-minus_one_ptid: invalid (checked by assert) - non-NULL target and non-minus_one_ptid: we delete all the entries associated to that tuple, this is done efficiently - a non-NULL target and minus_one_ptid: we delete all the entries associated to that target, whatever the ptid. This is the slightly annoying case, as we can't easily look up all items having this target in their key. I implemented it by walking the list, which is not ideal. The function regcache_thread_ptid_changed is called when a thread changes ptid. It is implemented efficiently using the map, although that's not very important: it is not called often, mostly when creating an inferior, on some specific platforms. Note: In hash_target_ptid, I am combining hash values from std::hash by summing them. I don't think it's ideal, since std::hash is just the identity function for base types. But I don't know what would be better to reduce the change of collisions. If anybody has a better idea, I'd be interested. This patch is a tiny bit from ROCm GDB [1] we would like to merge upstream. Laurent Morichetti gave be these performance numbers: The benchmark used is: time ./gdb --data-directory=data-directory /extra/lmoriche/hip/samples/0_Intro/bit_extract/bit_extract -ex "set pagination off" -ex "set breakpoint pending on" -ex "b bit_extract_kernel if \$_thread == 5" -ex run -ex c -batch It measures the time it takes to continue from a conditional breakpoint with 2048 threads at that breakpoint, one of them reporting the breakpoint. baseline: real 0m10.227s real 0m10.177s real 0m10.362s with patch: real 0m8.356s real 0m8.424s real 0m8.494s [1] https://github.com/ROCm-Developer-Tools/ROCgdb gdb/ChangeLog: * regcache.c (struct target_ptid): New struct. (hash_target_ptid): New struct. (target_ptid_regcache_map): New type. (regcaches): Change type to target_ptid_regcache_map. (get_thread_arch_aspace_regcache): Update to regcaches' new type. (regcache_thread_ptid_changed): Likewise. (registers_changed_ptid): Likewise. (regcaches_size): Likewise. (regcaches_test): Update. (regcache_thread_ptid_changed): Update. * gdbsupport/ptid.h (hash_ptid): New struct. Change-Id: Iabb0a1111707936ca111ddb13f3b09efa83d3402 |
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.