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| Started by | "Michael S. Tsirkin" <mst@redhat.com> |
|---|---|
| First post | 2016-01-12 23:20 +0100 |
| Last post | 2016-01-12 23:30 +0100 |
| Articles | 2 — 2 participants |
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[PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks "Michael S. Tsirkin" <mst@redhat.com> - 2016-01-12 23:20 +0100
Re: [PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks "H. Peter Anvin" <hpa@zytor.com> - 2016-01-12 23:30 +0100
| From | "Michael S. Tsirkin" <mst@redhat.com> |
|---|---|
| Date | 2016-01-12 23:20 +0100 |
| Subject | [PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks |
| Message-ID | <qQfsd-7eN-3@gated-at.bofh.it> |
mb() typically uses mfence on modern x86, but a micro-benchmark shows that it's 2 to 3 times slower than lock; addl $0,(%%e/rsp) that we use on older CPUs. So let's use the locked variant everywhere - helps keep the code simple as well. While I was at it, I found some inconsistencies in comments in arch/x86/include/asm/barrier.h I hope I'm not splitting this up too much - the reason is I wanted to isolate the code changes (that people might want to test for performance) from comment changes approved by Linus, from (so far unreviewed) comment change I came up with myself. Lightly tested on my system. Michael S. Tsirkin (3): x86: drop mfence in favor of lock+addl x86: drop a comment left over from X86_OOSTORE x86: tweak the comment about use of wmb for IO arch/x86/include/asm/barrier.h | 10 +++------- 1 file changed, 3 insertions(+), 7 deletions(-) -- MST
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| From | "H. Peter Anvin" <hpa@zytor.com> |
|---|---|
| Date | 2016-01-12 23:30 +0100 |
| Message-ID | <qQfBT-7kg-5@gated-at.bofh.it> |
| In reply to | #1307878 |
On 01/12/16 14:10, Michael S. Tsirkin wrote: > mb() typically uses mfence on modern x86, but a micro-benchmark shows that it's > 2 to 3 times slower than lock; addl $0,(%%e/rsp) that we use on older CPUs. > > So let's use the locked variant everywhere - helps keep the code simple as > well. > > While I was at it, I found some inconsistencies in comments in > arch/x86/include/asm/barrier.h > > I hope I'm not splitting this up too much - the reason is I wanted to isolate > the code changes (that people might want to test for performance) from comment > changes approved by Linus, from (so far unreviewed) comment change I came up > with myself. > > Lightly tested on my system. > > Michael S. Tsirkin (3): > x86: drop mfence in favor of lock+addl > x86: drop a comment left over from X86_OOSTORE > x86: tweak the comment about use of wmb for IO > I would like to get feedback from the hardware team about the implications of this change, first. -hpa
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