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.
> 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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