I took some time to put this to a test. The Go program here https://go.dev/play/p/378Zn_ZQNaz <https://go.dev/play/p/rNJNbek4ufm> uses a VERY short holding of the lock - but a large % of runtime holding the lock.
(You can’t run it on the Playground because of the length of time). You can comment/uncomment the lines 28-31 to test the different mutexes, It simulates a common system scenario (most web services) - lots of readers of the cache, but the cache is updated infrequently. On my machine the RWMutex is > 50% faster - taking 22 seconds vs 47 seconds using a simple Mutex. It is easy to understand why - you get no parallelization of the readers when using a simple Mutex. > On Jan 30, 2023, at 8:29 PM, Ian Lance Taylor <i...@golang.org> wrote: > > On Mon, Jan 30, 2023 at 4:42 PM Robert Engels <reng...@ix.netcom.com> wrote: >> >> Yes but only for a single reader - any concurrent reader is going to >> park/deschedule. > > If we are talking specifically about Go, then it's more complex than > that. In particular, the code will spin briefly trying to acquire the > mutex, before queuing. > >> There’s a reason RW locks exist - and I think it is pretty common - but >> agree to disagree :) > > Sure: read-write locks are fine and appropriate when the program holds > the read lock for a reasonably lengthy time. As I said, my analysis > only applies when code holds the read lock briefly, as is often the > case for a cache. > > Ian > > >>> On Jan 30, 2023, at 6:23 PM, Ian Lance Taylor <i...@golang.org> wrote: >>> >>> 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. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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