Hello,
On Wed, Nov 8, 2023 at 9:12 AM Joe Mario <jmario@xxxxxxxxxx> wrote:
>
> Hi Namhyung:
>
> I've been playing with your datatype profile patch and it looks really promising.
> I think it would be a big help if it could be integrated into perf c2c.
Great! Yeah, I think we can collaborate on it.
>
> Perf c2c gives a great insight into what's contributing to cpu cacheline contention, but it
> can be difficult to understand the output. Having visuals with your datatype profile output
> would be a big help.
Exactly.
>
> I have a simple test program with readers and writers tugging on the data below:
>
> uint64_t hotVar;
> typedef struct __foo {
> uint64_t m1;
> uint64_t m2;
> uint64_t m3;
> } FOO;
>
> The rest of this reply looks at both your datatype output and c2c to see where they
> might compliment each other.
>
>
> When I run perf with your patches on a simple program to cause contention on the above data, I get the following:
>
> # perf mem record --ldlat=1 --all-user -- ./tugtest -r3 -r5 -r7 -r9 -r11 -w10 -w12 -w14 -w16 -b5 -H2000000
> # perf report -s type,typeoff --hierarchy --stdio
>
> # Samples: 26K of event 'cpu/mem-loads,ldlat=1/P'
> # Event count (approx.): 2958226
> #
> # Overhead Data Type / Data Type Offset
> # ........... ............................
> #
> 54.50% int
> 54.50% int +0 (no field)
> 23.21% long int
> 23.21% long int +0 (no field)
> 18.30% struct __foo
> 9.57% struct __foo +8 (m2)
> 8.73% struct __foo +0 (m1)
> 3.86% long unsigned int
> 3.86% long unsigned int +0 (no field)
> <snip>
>
> # Samples: 30K of event 'cpu/mem-stores/P'
> # Event count (approx.): 33880197
> #
> # Overhead Data Type / Data Type Offset
> # ........... ............................
> #
> 99.85% struct __foo
> 70.48% struct __foo +0 (m1)
> 29.34% struct __foo +16 (m3)
> 0.03% struct __foo +8 (m2)
> 0.09% long unsigned int
> 0.09% long unsigned int +0 (no field)
> 0.06% (unknown)
> 0.06% (unknown) +0 (no field)
> <snip>
>
> Then I run perf annotate with your patches, and I get the following:
>
> # perf annotate --data-type
>
> Annotate type: 'long int' in /home/joe/tugtest/tugtest (2901 samples):
> ============================================================================
> samples offset size field
> 2901 0 8 long int ;
>
> Annotate type: 'struct __foo' in /home/joe/tugtest/tugtest (5593 samples):
> ============================================================================
> samples offset size field
> 5593 0 24 struct __foo {
> 2755 0 8 uint64_t m1;
> 2838 8 8 uint64_t m2;
> 0 16 8 uint64_t m3;
> };
>
> Now when I run that same simple test using perf c2c, and I focus on the cachline that the struct and hotVar reside in, I get:
>
> # perf c2c record --all-user -- ./tugtest -r3 -r5 -r7 -r9 -r11 -w10 -w12 -w14 -w16 -b5 -H2000000
> # perf c2c report -NNN --stdio
> # <snip>
> #
> # ----- HITM ----- ------- Store Refs ------ ------ Data address ------ ---------- cycles ---------- Total cpu Shared
> # Num RmtHitm LclHitm L1 Hit L1 Miss N/A Offset Node PA cnt Code address rmt hitm lcl hitm load records cnt Symbol Object Source:Line Node{cpu list}
> #.... ....... ....... ....... ....... ....... ............ .... ...... ............ ........ ........ ........ ....... ..... .......... ....... ............. ....
> #
> ---------------------------------------------------------------
> 0 1094 2008 17071 13762 0 0x406100
> ---------------------------------------------------------------
> 0.00% 0.20% 0.00% 0.00% 0.00% 0x8 1 1 0x401355 0 978 1020 2962 4 [.] writer tugtest tugtest.c:129 0{10,12,14,16}
> 0.00% 0.00% 0.12% 0.02% 0.00% 0x8 1 1 0x401360 0 0 0 23 4 [.] writer tugtest tugtest.c:129 0{10,12,14,16}
> 68.10% 60.26% 0.00% 0.00% 0.00% 0x10 1 1 0x401505 2181 1541 1393 5813 5 [.] reader tugtest tugtest.c:163 1{3,5,7,9,11}
> 31.63% 39.34% 0.00% 0.00% 0.00% 0x10 1 1 0x401331 1242 1095 936 3393 4 [.] writer tugtest tugtest.c:127 0{10,12,14,16}
> 0.00% 0.00% 40.03% 40.25% 0.00% 0x10 1 1 0x40133c 0 0 0 12372 4 [.] writer tugtest tugtest.c:127 0{10,12,14,16}
> 0.27% 0.15% 0.00% 0.00% 0.00% 0x18 1 1 0x401343 834 1136 1032 2930 4 [.] writer tugtest tugtest.c:128 0{10,12,14,16}
> 0.00% 0.05% 0.00% 0.00% 0.00% 0x18 1 1 0x40150c 0 933 1567 5050 5 [.] reader tugtest tugtest.c:163 1{3,5,7,9,11}
> 0.00% 0.00% 0.06% 0.00% 0.00% 0x18 1 1 0x40134e 0 0 0 10 4 [.] writer tugtest tugtest.c:128 0{10,12,14,16}
> 0.00% 0.00% 59.80% 59.73% 0.00% 0x20 1 1 0x401516 0 0 0 18428 5 [.] reader tugtest tugtest.c:163 1{3,5,7,9,11}
>
> With the above c2c output, we can see:
> - the hottest contended addresses, and the load latencies they caused.
> - the cacheline offset for the contended addresses.
> - the cpus and numa nodes where the accesses came from.
> - the cacheline alignment for the data of interest.
> - the number of cpus and threads concurrently accessing each address.
> - the breakdown of reads causing HITM (contention) and writes hitting or missing the cacheline.
> - the object name, source line and line number for where the accesses occured.
> - the numa node where the data is allocated.
> - the number of physical pages the virtual addresses were mapped to (e.g. numa_balancing).
>
> What would really help the c2c output be more usable is if it had a better visual to it.
> It's likely the current c2c output can be trimmed a bit.
>
> Here's one idea that incorporates your datatype info, though I'm sure there are better ways, as this may get unwieldy.:
>
> # ----- HITM ----- ------- Store Refs ------ ------ Data address ------ ---------- cycles ---------- Total cpu Shared
> # Num RmtHitm LclHitm L1 Hit L1 Miss N/A Offset Node PA cnt Code address rmt hitm lcl hitm load records cnt Symbol Object Source:Line Node{cpu list}
> #.... ....... ....... ....... ....... ....... ............ .... ...... ............ ........ ........ ........ ....... ..... .......... ....... ............. ....
> #
> ---------------------------------------------------------------
> 0 1094 2008 17071 13762 0 0x406100
> ---------------------------------------------------------------
> uint64_t hotVar: tugtest.c:38
> 0.00% 0.20% 0.00% 0.00% 0.00% 0x8 1 1 0x401355 0 978 1020 2962 4 [.] writer tugtest tugtest.c:129 0{10,12,14,16}
> 0.00% 0.00% 0.12% 0.02% 0.00% 0x8 1 1 0x401360 0 0 0 23 4 [.] writer tugtest tugtest.c:129 0{10,12,14,16}
> struct __foo uint64_t m1: tugtest.c:39
> 68.10% 60.26% 0.00% 0.00% 0.00% 0x10 1 1 0x401505 2181 1541 1393 5813 5 [.] reader tugtest tugtest.c:163 1{3,5,7,9,11}
> 31.63% 39.34% 0.00% 0.00% 0.00% 0x10 1 1 0x401331 1242 1095 936 3393 4 [.] writer tugtest tugtest.c:127 0{10,12,14,16}
> 0.00% 0.00% 40.03% 40.25% 0.00% 0x10 1 1 0x40133c 0 0 0 12372 4 [.] writer tugtest tugtest.c:127 0{10,12,14,16}
> struct __foo uint64_t m2: tugtest.c:40
> 0.27% 0.15% 0.00% 0.00% 0.00% 0x18 1 1 0x401343 834 1136 1032 2930 4 [.] writer tugtest tugtest.c:128 0{10,12,14,16}
> 0.00% 0.05% 0.00% 0.00% 0.00% 0x18 1 1 0x40150c 0 933 1567 5050 5 [.] reader tugtest tugtest.c:163 1{3,5,7,9,11}
> 0.00% 0.00% 0.06% 0.00% 0.00% 0x18 1 1 0x40134e 0 0 0 10 4 [.] writer tugtest tugtest.c:128 0{10,12,14,16}
> struct __foo uint64_t m3: tugtest.c:41
> 0.00% 0.00% 59.80% 59.73% 0.00% 0x20 1 1 0x401516 0 0 0 18428 5 [.] reader tugtest tugtest.c:163 1{3,5,7,9,11}
>
> And then it would be good to find a clean way to incorporate your sample counts.
I'm not sure we can get the exact source line for the data type/fields.
Of course, we can aggregate the results for each field. Actually you
can use `perf report -s type,typeoff,symoff --hierarchy` for something
similar. :)
>
> On a related note, is there a way the accesses could be broken down into read counts
> and write counts? That, with the above source line info for all the accesses,
> helps to convey a picture of "the affinity of the accesses".
Sure, perf report already supports showing events in a group
together. You can use --group option to force grouping
individual events. perf annotate with --data-type doesn't have
that yet. I'll update it in v2.
>
> For example, while it's normally good to separate read-mostly data from hot
> written data, if the reads and writes are done together in the same block of
> code by the same thread, then keeping the two data symbols in the same cacheline
> could be a win. I've seen this often. Your datatype info might be able to
> make these affinities more visible to the user.
>
> Thanks for doing this. This is great.
> Joe
Thanks for your feedback!
Namhyung
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