That only works if what it is pointing to is cheap to copy. If it is a large multi-layer structure a RW lock is usually more efficient.
> On Feb 4, 2023, at 5:19 PM, Ian Lance Taylor <i...@golang.org> wrote: > > On Sat, Feb 4, 2023 at 3:11 PM Robert Engels <reng...@ix.netcom.com> wrote: >> >> I think with server processes - with possibly 100k+ connections - the >> contention on a “read mainly” cache is more than you think. This test only >> uses 500 readers with little work to simulate the 100k case. > > Not to get too far into the weeds, but if I were expecting that kind > of load I would use an atomic.Pointer anyhow, rather than any sort of > mutex. > > Ian > >>>> On Feb 4, 2023, at 4:59 PM, Ian Lance Taylor <i...@golang.org> wrote: >>> >>> On Sat, Feb 4, 2023 at 8:49 AM robert engels <reng...@ix.netcom.com> wrote: >>>> >>>> I took some time to put this to a test. The Go program here >>>> https://go.dev/play/p/378Zn_ZQNaz 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. >>> >>> Thanks for the benchmark. You're right: if you have hundreds of >>> goroutines doing nothing but acquiring a read lock, then an RWMutex >>> can be faster. They key there is that there are always multiple >>> goroutines waiting for the lock. >>> >>> I still stand by my statement for more common use cases. >>> >>> Ian >>> >>> >>>> 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. >>> 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/CAOyqgcV-7RfjXakYkc-pVJHPwhkaTLXky0mOMXbhqpcXLGwp2Q%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/CAOyqgcVgOfcSr%2BvzTKGMpicw1hbD6bzrB5yZhOn-sYGW81b6tw%40mail.gmail.com. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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