Date: Fri, 18 Mar 2016 10:37:55 -0700 From: John Baldwin <jhb@freebsd.org> To: Adrian Chadd <adrian.chadd@gmail.com> Cc: Konstantin Belousov <kostikbel@gmail.com>, Stanislav Sedov <stas@freebsd.org>, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: Peformance issues with r278325 Message-ID: <3277812.DVsZx4uMun@ralph.baldwin.cx> In-Reply-To: <CAJ-VmonUdHRJ0jcFphqXi1w2PwbDycLG190Yd5z8JQWGxW_1iQ@mail.gmail.com> References: <FA50A68E-7F3D-4361-8A8A-EB7F97EF3D00@FreeBSD.org> <8403291.NqUNo0Qq5W@ralph.baldwin.cx> <CAJ-VmonUdHRJ0jcFphqXi1w2PwbDycLG190Yd5z8JQWGxW_1iQ@mail.gmail.com>
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On Thursday, March 17, 2016 10:45:39 AM Adrian Chadd wrote: > On 17 March 2016 at 10:23, John Baldwin <jhb@freebsd.org> wrote: > > > > > I had not expected this commit to have this impact, but Konstantin is correct. > > I wonder if simply reducing the DELAY() from 5 down to 1 or so would be > > sufficient? (Note that you need to adjust the prior loop to use += 1 instead > > of += 5 in that case.) > > > > Note that DETECT_DEADLOCK is not enabled by default, so the AFTER_SPIN case > > (which waits for an IPI just sent to be delivered) shouldn't be enabled (and > > in fact I'd like to just remove that code entirely). This means that only > > BEFORE_SPIN should be spinning, and it should only be spinning if a CPU sends > > IPIs back to back such that the previous IPI is still pending (not yet > > delivered) when the CPU wants to send another IPI. > > > > We can probably assume a TSC if we have SMP, so if changing the delay from 5 > > to 1 doesn't work we can try just using the TSC directly to control the > > spin length and go back to using a simple pause. > > > > I have an old set of changes that might also be interesting that permit > > TLB shootdown IPI handlers to run while spinlocks are held (by using cr8/TPR > > to control interrupts when a spinlock is held instead of disabling all > > interrupts). I haven't found a workload where that helped yet. However, > > yours might be an interesting workload to try those changes out on. > > Do you think it's worth just reverting it for now just so it lands in > 10.3-RELEASE? Probably. If the '1' change fixes it that is a simple test, otherwise we can revert in 10.x. I think I'll likely just convert it to use a direct TSC delay loop always in HEAD (assuming that verifies ok in testing as well). -- John Baldwin
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