On 03/06/23 16:40, Mike Kravetz wrote: > On 03/06/23 19:15, Peter Xu wrote: > > On Mon, Mar 06, 2023 at 03:57:30PM -0800, Mike Kravetz wrote: > > > > > > Just wondering if there is anything better or more selective that can be > > > done? Does it make sense to have THP backed stacks by default? If not, > > > who would be best at disabling? A couple thoughts: > > > - The kernel could disable huge pages on stacks. libpthread/glibc pass > > > the unused flag MAP_STACK. We could key off this and disable huge pages. > > > However, I'm sure there is somebody somewhere today that is getting better > > > performance because they have huge pages backing their stacks. > > > - We could push this to glibc/libpthreads and have them use > > > MADV_NOHUGEPAGE on thread stacks. However, this also has the potential > > > of regressing performance if somebody somewhere is getting better > > > performance due to huge pages. > > > > Yes it seems it's always not safe to change a default behavior to me. > > > > For stack I really can't tell why it must be different here. I assume the > > problem is the wasted space and it exaggerates easily with N-threads. But > > IIUC it'll be the same as thp to normal memories iiuc, e.g., there can be a > > per-thread mmap() of 2MB even if only 4K is used each, then if such mmap() > > is populated by THP for each thread there'll also be a huge waste. I may be alone in my thinking here, but it seems that stacks by their nature are not generally good candidates for huge pages. I am just thinking about the 'normal' use case where stacks contain local function data and arguments. Am I missing something, or are huge pages really a benefit here? Of course, I can imagine some thread with a large amount of frequently accessed data allocated on it's stack which could benefit from huge pages. But, this seems to be an exception rather than the rule. I understand the argument that THP always means always and everywhere. It just seems that thread stacks may be 'special enough' to consider disabling by default. -- Mike Kravetz
