FWIW, Using an RCU (https://en.wikipedia.org/wiki/Read-copy-update) algorithm is often the best choice for caches where reads happen several orders of magnitude more often than writes. RCU usually avoids the need for any mutex by readers. I was working at Sequent Computer Systems when this was implemented for the network ARP and routing tables and was friends with the engineers who designed and patented RCU. I remember reading their patent and looking at the kernel implementation and thinking to myself how brilliant the concept was.
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 <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. > 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/2AA31B8D-8A35-4894-AC1F-48E830297887%40ix.netcom.com > <https://groups.google.com/d/msgid/golang-nuts/2AA31B8D-8A35-4894-AC1F-48E830297887%40ix.netcom.com?utm_medium=email&utm_source=footer> > . > -- Kurtis Rader Caretaker of the exceptional canines Junior and Hank -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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