On Mon, Jan 30, 2023 at 1:00 PM Robert Engels <reng...@ix.netcom.com> wrote: > > Pure readers do not need any mutex on the fast path. It is an atomic CAS - > which is faster than a mutex as it allows concurrent readers. On the slow > path - fairness with a waiting or active writer - it degenerates in > performance to a simple mutex. > > The issue with a mutex is that you need to acquire it whether reading or > writing - this is slow…. (at least compared to an atomic cas)
The fast path of a mutex is also an atomic CAS. Ian > > On Jan 30, 2023, at 2:24 PM, Ian Lance Taylor <i...@golang.org> wrote: > > > > On Mon, Jan 30, 2023 at 11:26 AM Robert Engels <reng...@ix.netcom.com> > > wrote: > >> > >> I don’t think that is true. A RW lock is always better when the reader > >> activity is far greater than the writer - simply because in a good > >> implementation the read lock can be acquired without blocking/scheduling > >> activity. > > > > The best read lock implementation is not going to be better than the > > best plain mutex implementation. And with current technology any > > implementation is going to require atomic memory operations which > > require coordinating cache lines between CPUs. If your reader > > activity is so large that you get significant contention on a plain > > mutex (recalling that we are assuming the case where the operations > > under the read lock are quick) then you are also going to get > > significant contention on a read lock. The effect is that the read > > lock isn't going to be faster anyhow in practice, and your program > > should probably be using a different approach. > > > > Ian > > > >>>> On Jan 30, 2023, at 12:49 PM, Ian Lance Taylor <i...@golang.org> wrote: > >>> > >>> On Sun, Jan 29, 2023 at 6:34 PM Diego Augusto Molina > >>> <diegoaugustomol...@gmail.com> wrote: > >>>> > >>>> From times to times I write a scraper or some other tool that would > >>>> authenticate to a service and then use the auth result to do stuff > >>>> concurrently. But when auth expires, I need to synchronize all my > >>>> goroutines and have a single one do the re-auth process, check the > >>>> status, etc. and then arrange for all goroutines to go back to work > >>>> using the new auth result. > >>>> > >>>> To generalize the problem: multiple goroutines read a cached value that > >>>> expires at some point. When it does, they all should block and some I/O > >>>> operation has to be performed by a single goroutine to renew the cached > >>>> value, then unblock all other goroutines and have them use the new value. > >>>> > >>>> I solved this in the past in a number of ways: having a single goroutine > >>>> that handles the cache by asking it for the value through a channel, > >>>> using sync.Cond (which btw every time I decide to use I need to > >>>> carefully re-read its docs and do lots of tests because I never get it > >>>> right at first). But what I came to do lately is to implement an > >>>> upgradable lock and have every goroutine do: > >>> > >>> > >>> We have historically rejected this kind of adjustable lock. There is > >>> some previous discussion at https://go.dev/issue/4026, > >>> https://go.dev/issue/23513, https://go.dev/issue/38891, > >>> https://go.dev/issue/44049. > >>> > >>> For a cache where checking that the cached value is valid (not stale) > >>> and fetching the cached value is quick, then in general you will be > >>> better off using a plain Mutex rather than RWMutex. RWMutex is more > >>> complicated and therefore slower. It's only useful to use an RWMutex > >>> when the read case is both contested and relatively slow. If the read > >>> case is fast then the simpler Mutex will tend to be faster. And then > >>> you don't have to worry about upgrading the lock. > >>> > >>> Ian > >>> > >>> -- > >>> You received this message because you are subscribed to the Google Groups > >>> "golang-nuts" group. > >>> To unsubscribe from this group and stop receiving emails from it, send an > >>> email to golang-nuts+unsubscr...@googlegroups.com. > >>> To view this discussion on the web visit > >>> https://groups.google.com/d/msgid/golang-nuts/CAOyqgcXNVFkc5H-L6K4Mt81gB6u91Ja07hob%3DS8Qwgy2buiQjQ%40mail.gmail.com. > > > > -- > > You received this message because you are subscribed to the Google Groups > > "golang-nuts" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to golang-nuts+unsubscr...@googlegroups.com. > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/golang-nuts/CAOyqgcWJ%2BLPOoTk9H7bxAj8_dLsuhgOpy_bZZrGW%3D%2Bz6N%3DrX-w%40mail.gmail.com. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAOyqgcVLzkTgiYqw%2BWh6pTFX74X-LYoyPFK5bkX7T8J8j3mb%3Dg%40mail.gmail.com.
