On Wed, Mar 29, 2023 at 9:27 PM Bagas Sanjaya <bagasdotme@xxxxxxxxx> wrote: > > On Wed, Mar 29, 2023 at 03:33:22PM +0800, Feng Tang wrote: > > +False sharing hurting performance cases are seen more frequently with > > +core count increasing. Because of these detrimental effects, many > > +patches have been proposed across variety of subsystems (like > > +networking and memory management) and merged. Some common mitigations > > +(with examples) are: > > + > > +* Separate hot global data in its own dedicated cache line, even if it > > + is just a 'short' type. The downside is more consumption of memory, > > + cache line and TLB entries. > > + > > + - Commit 91b6d3256356 ("net: cache align tcp_memory_allocated, tcp_sockets_allocated") > > + > > +* Reorganize the data structure, separate the interfering members to > > + different cache lines. One downside is it may introduce new false > > + sharing of other members. > > + > > + - Commit 802f1d522d5f ("mm: page_counter: re-layout structure to reduce false sharing") > > + > > +* Replace 'write' with 'read' when possible, especially in loops. > > + Like for some global variable, use compare(read)-then-write instead > > + of unconditional write. For example, use:: > > + > > + if (!test_bit(XXX)) > > + set_bit(XXX); > > + > > + instead of directly "set_bit(XXX);", similarly for atomic_t data. > "... The similar technique is also applicable to atomic_t data". > > But how? > > > + > > + - Commit 7b1002f7cfe5 ("bcache: fixup bcache_dev_sectors_dirty_add() multithreaded CPU false sharing") > > + - Commit 292648ac5cf1 ("mm: gup: allow FOLL_PIN to scale in SMP") > > + > > +* Turn hot global data to 'per-cpu data + global data' when possible, > > + or reasonably increase the threshold for syncing per-cpu data to > > + global data, to reduce or postpone the 'write' to that global data. > > + > > + - Commit 520f897a3554 ("ext4: use percpu_counters for extent_status cache hits/misses") > > + - Commit 56f3547bfa4d ("mm: adjust vm_committed_as_batch according to vm overcommit policy") > > + > > Here's what I mean by bridging conjunctions to example commits as I reviewed > in v1 [1]: > This is too much and unneeded nitpicking. The patch looks good as is.