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. 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