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) > 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/9AC5447A-2EAC-4FD0-9AEE-96EA517ABB94%40ix.netcom.com.
