Date: Wed, 25 Feb 2015 00:29:29 -0800 From: Harrison Grundy <harrison.grundy@astrodoggroup.com> To: freebsd-arch@freebsd.org Subject: Re: locks and kernel randomness... Message-ID: <54ED87E9.8030706@astrodoggroup.com> In-Reply-To: <54ED80BD.1080603@freebsd.org> References: <20150224015721.GT74514@kib.kiev.ua> <54EBDC1C.3060007@astrodoggroup.com> <20150224024250.GV74514@kib.kiev.ua> <DD06E2EA-68D6-43D7-AA17-FB230750E55A@bsdimp.com> <20150224174053.GG46794@funkthat.com> <54ECBD4B.6000007@freebsd.org> <20150224182507.GI46794@funkthat.com> <54ECEA43.2080008@freebsd.org> <20150224231921.GQ46794@funkthat.com> <CAHM0Q_NhUpr_HJZZcAEoZ_hNvZKcVzUBH-7LALsbkgqjLimA7A@mail.gmail.com> <20150225002301.GS46794@funkthat.com> <54ED80BD.1080603@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On 02/24/15 23:58, Alfred Perlstein wrote: > > On 2/24/15 7:23 PM, John-Mark Gurney wrote: >> K. Macy wrote this message on Tue, Feb 24, 2015 at 15:33 -0800: >>>> If someone does find a performance issue w/ my patch, I WILL >>>> work with them on a solution, but I will not work w/ people >>>> who make unfounded claims about the impact of this work... >>>> >>> <shrug> ... The concerns may be exaggerated, but they aren't >>> unfounded. Not quite the same thing, but no one wants to spend >>> the >> Till someone shows me code in the kernel tree where this is even >> close to a performance problem, it is unfounded... I've asked, >> and no one has >> >>> cycles doing a SHA256 because it's "The Right Thing"(tm) when >>> their use case only requires a fletcher2. >> Depends upon what you're doing.. I haven't proposed changing >> ZFS's default to sha256, so stop w/ the false equivalences... >> >>> If it doesn't already exist, it might also be worth looking in >>> to a more scalable CSPRNG implementation not requiring locking >>> in the common case. For example, each core is seeded separately >>> periodically so that has a private pool that is protected by a >>> critical section. The private pool would be regularly refreshed >>> by cpu-local callout. Thus, a lock would only be acquired if >>> the local entropy were depleted. >> I'm not discussing this until you read and reply to my original >> email, since it's clear that my original email's contents has >> been ignored in this thread... >> > What is final proposal? More spinlocks? That is not a good idea. > > Doing a single buildworld is not enough. Ask netflix or someone > with a real load of 1000s of threads/processing to do testing for > you if you truly want to touch scheduler. sched_ule runs this code once every .5 to 1.5 seconds, depending on the value of random, so using a CSPRNG there wouldn't actually be noticeable. (We're talking about a few thousand cycles, when the existing implementation has to make a remote memory read/write numpackages-1/numpackages percent of the time, which costs tens of thousands of cycles. Switching to a per-CPU CSPRNG is actually faster in those cases.) That being said, I believe the plan is to remove random() from sched_ule entirely. It doesn't need it to perform the balancing, and we can just use the LCG from cpu_search, if get_cyclecount isn't viable. --- Harrison
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?54ED87E9.8030706>