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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