Messages in this thread | | | Subject | Re: [PATCH 04/15] ftrace: pack event structures. | From | Steven Rostedt <> | Date | Wed, 09 Mar 2011 08:01:55 -0500 |
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On Fri, 2010-12-03 at 16:13 -0800, David Sharp wrote: > Ftrace event structures have a 12-byte struct trace_entry at the beginning. > If the structure is aligned, this means that if the first field is 64-bits, > there will be 4 bytes of padding. Ironically, due to the 4-byte ringbuffer > header, this will make 64-bit writes unaligned, if the ring buffer position > is currently 64-bit aligned: > 4(rb)+12(ftrace)+4(pad) = 20; 20%8 = 4
Actually the better answer is: It's time to nuke the lock_depth field. That was added temporarily in helping to remove the BKL, and now that it's pretty much gone, I say we need to nuke it. That will remove 4 bytes from the 12 byte ftrace header giving us a 8 byte header. Much nicer :)
I'll do that today. Thanks!
-- Steve
> > Adding __attribute__((packed)) to the event structures removes the extra > space from the trace events, and actually improves alignment of trace > events with a first field that is 64-bits. > > About 65 tracepoints have a 4-byte pad at offset 12: > # find events -name format | xargs -n1 awk ' > $1=="name:" {name=$2} > $1=="format:"{FS="\t"} > $3=="offset:12;" && $4=="size:4;"{okay=1} > $3=="offset:16;" && !okay {print name}' | wc -l > 65 > > With all 'syscalls' and 'timer' events enabled, this results in a 5% > improvement in a simple 512MB read benchmark with warm caches.
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