Michal Hocko <mhocko@xxxxxxxx> writes: > On Mon 10-07-23 14:53:24, Huang Ying wrote: >> The page allocation performance requirements of different workloads >> are usually different. So, we often need to tune PCP (per-CPU >> pageset) high to optimize the workload page allocation performance. >> Now, we have a system wide sysctl knob (percpu_pagelist_high_fraction) >> to tune PCP high by hand. But, it's hard to find out the best value >> by hand. And one global configuration may not work best for the >> different workloads that run on the same system. One solution to >> these issues is to tune PCP high of each CPU automatically. >> >> This patch adds the framework for PCP high auto-tuning. With it, >> pcp->high will be changed automatically by tuning algorithm at >> runtime. Its default value (pcp->high_def) is the original PCP high >> value calculated based on low watermark pages or >> percpu_pagelist_high_fraction sysctl knob. To avoid putting too many >> pages in PCP, the original limit of percpu_pagelist_high_fraction >> sysctl knob, MIN_PERCPU_PAGELIST_HIGH_FRACTION, is used to calculate >> the max PCP high value (pcp->high_max). > > It would have been very helpful to describe the basic entry points to > the auto-tuning. AFAICS the central place of the tuning is tune_pcp_high > which is called from the freeing path. Why? Is this really a good place > considering this is a hot path? What about the allocation path? Isn't > that a good spot to watch for the allocation demand? Yes. The main entry point to the auto-tuning is tune_pcp_high(). Which is called from the freeing path because pcp->high is only used by page freeing. It's possible to call it in allocation path instead. The drawback is that the pcp->high may be updated a little later in some situations. For example, if there are many page freeing but no page allocation for quite long time. But I don't think this is a serious problem. > Also this framework seems to be enabled by default. Is this really > desirable? What about workloads tuning the pcp batch size manually? > Shouldn't they override any auto-tuning? In the current implementation, the pcp->high will be tuned between original pcp high (default or tuned manually) and the max pcp high (via MIN_PERCPU_PAGELIST_HIGH_FRACTION). So the high value tuned manually is respected at some degree. So you think that it's better to disable auto-tuning if PCP high is tuned manually? Best Regards, Huang, Ying
