On Sun, Mar 18, 2018 at 4:44 AM, Radoslaw Zarzynski <rzarzyns@xxxxxxxxxx> wrote: > Hello, > > some time ago I've promised to Greg a post highlighting performance > issue that was discovered couple weeks ago: many calls to the costly > `md_config_t::get_val<T>` on OSD's hot paths. > > The problem is that the method really needs to acquire a mutex and > isn't supposed to be used in performance-critical situations. > For handling them the config rework designated the `md_config_obs_t` > interface. Implementing it allows clients to locally cache setting's value > giving very fast access. > > Though, it's understandable that one, single call is much easier to use > as it doesn't require any boilerplate. Because of that a grain of sugar > was added: `md_config_cacher_t` [1]. It handles registration/value > update/unregistration simplifying the observer usage in typical cases. > > The idea [2] is to make `md_config_cacher_t<SettingT>` a member > of a longly living instance (e.g. shard), then initialize it with a name of > interesting configurable (+ a reference to md_config_t). Obtaining > the cached value is simplified thanks to the implicit conversion to > `SettingT`. > > At the moment there is a limitation on `SettingT`: it can't be std::string > due to the std::atomic<> config cacher uses inside. > > Surely, there is no business in converting everything to use the config > observer pattern. `get_val<T>` is perfectly fine during initialization, > error handling and generally on all infrequent paths. Only the hottest > ones might be an issue. Excellent! This is a very useful compliment to the new-style config stuff. John > Best regards. > Radosław Zarzyński > > > `md_config_cacher_t` implementation: > [1] https://github.com/ceph/ceph/blob/master/src/common/config_cacher.h > > Example of usage: > [2] https://github.com/ceph/ceph/pull/20320/commits/248750fea441efd16c1a431617cfbd1f868a6eeb > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
