Thanks for pointing me to this patch. I guess the try-locks still does not solve the overhead of multiple readers contending the counter. It just provides a non-blocking version of the same algorithm.
The relock function looks interesting. But it would be helpful if any special use case or example is given. Specifically, who should call this function, the reader or the writer? Leonid, could you provide more context? The TLRW example I gave is potentially a better rw-lock algorithm. Paper Reference: D. Dice and N Shavit. "TLRW: return of the read-write lock". In such scheme, readers won't contend the same reader counter which introduces heavy cache bouncing that Stephen mentioned. Maybe we should introduce similar algorithm into rte_rwlock library. >-----Original Message----- >From: Thomas Monjalon [mailto:tho...@monjalon.net] >Sent: Thursday, July 12, 2018 1:30 PM >To: Wang, Yipeng1 <yipeng1.w...@intel.com>; Stephen Hemminger ><step...@networkplumber.org> >Cc: dev@dpdk.org; De Lara Guarch, Pablo <pablo.de.lara.gua...@intel.com>; >Richardson, Bruce <bruce.richard...@intel.com>; >honnappa.nagaraha...@arm.com; vgu...@caviumnetworks.com; >brijesh.s.si...@gmail.com; Wang, Ren <ren.w...@intel.com>; >Gobriel, Sameh <sameh.gobr...@intel.com>; Tai, Charlie <charlie....@intel.com> >Subject: Re: [dpdk-dev] [PATCH v5 5/8] hash: add read and write concurrency >support > >12/07/2018 03:22, Wang, Yipeng1: >> From: Stephen Hemminger [mailto:step...@networkplumber.org] >> >> > For small windows, reader-writer locks are slower than a spin lock >> > because there are more cache bounces. >> >> Hi, Stephen, >> >> You are correct and we understand that spinlock might be slightly faster >> than counter based rwlock in this case. However, the >counter based rwlock is the exception path when TSX fails. >> >> If performance of this exception path is a big concern, a more optimal >> read-write lock scheme (e.g. TLRW) should be introduced into >rte_rwlock in the future. > >Something like this? > eal/rwlocks: Try read/write and relock write to read locks added > https://patches.dpdk.org/patch/40254/ > >
