Cgroup2: Reduce allocations for `manager.Stat` #281

dcantah · 2023-04-07T12:14:24Z

Related to #280. There's still some spots we can fix, as the majority of allocs are occurring in bufio.Scan in readKVStatsFile. cc @kolyshkin @manugupt1 seeming as how we were all there for the last one

This change works towards bringing down allocations for manager.Stat(). There's quite a few things we can do:

Swap to opening a file and issuing a single read for uint64/"max" values. Previously we were doing os.ReadFile's which returns a byte slice, so it needs to allocate.
Sizing the map we store {controller}.stat values in. We store 40+ stats in this map and the default bucket size for Go seems to be smaller than this, so we'd incur a couple allocs at runtime when adding these.
Finally, and funnily enough the biggest win, was just not calling f.Readdir(-1) for hugeTlb stats, and swapping this for a trick the runc cgroup code does. The number of allocs shaved off here is tied to the number of files in the cgroup directory
as f.Readdir(-1) was being called everytime we went to collect HugeTlb stats (it returns a slice of interfaces, that all we were doing is using the Name() method of anyways). To optimize this we can take advantage of the fact that we know exactly what files we need to open as they're fixed, the only variable portion is the hugepage sizes available on the host. To get around this we can compute the hugepage sizes in a sync.Once, and all future manager.Stat calls can reap the benefits as we don't need to Readdir() the entire cgroup path to search for hugetlb.pagesize.foobar's.

All in all on Go 1.19.4 this drops allocations in Stat from 322 to 198

benchstat ~/benchstat/old.txt ~/benchstat/new.txt 
goos: linux
goarch: amd64
pkg: github.com/containerd/cgroups/v3/cgroup2
cpu: AMD Ryzen 9 5900X 12-Core Processor            
        │ /home/dcantah/benchstat/old.txt │   /home/dcantah/benchstat/new.txt   │
        │             sec/op              │   sec/op     vs base                │
Stat-24                      112.29µ ± 3%   65.10µ ± 6%  -42.02% (p=0.000 n=10)

        │ /home/dcantah/benchstat/old.txt │   /home/dcantah/benchstat/new.txt    │
        │              B/op               │     B/op      vs base                │
Stat-24                      41.40Ki ± 0%   22.26Ki ± 0%  -46.23% (p=0.000 n=10)

        │ /home/dcantah/benchstat/old.txt │  /home/dcantah/benchstat/new.txt   │
        │            allocs/op            │ allocs/op   vs base                │
Stat-24                        322.0 ± 0%   198.0 ± 0%  -38.51% (p=0.000 n=10)

cgroup2/manager.go

cgroup2/utils_test.go

cgroup2/utils.go

kolyshkin · 2023-04-07T17:11:39Z

I suggest making readHugeTlbStats changes related to readdirnames as a separate commit.

dcantah · 2023-04-07T19:02:54Z

@kolyshkin Made a separate commit here for the hugeTlb changes, thanks for the quick review!

kolyshkin · 2023-04-07T22:20:44Z

cgroup2/manager.go

+	// Sizing this avoids an allocation to increase the map at runtime;
+	// currently the default bucket size is 8 and we put 40+ elements
+	// in it so we'd always end up allocating.
+	out := make(map[string]string, 50)


perhaps have the size as 0 and capacity as 64?

You can't set the capacity of a map or chan, only slices

Right, my bad, ENEEDCOFFEE

kolyshkin · 2023-04-07T22:42:05Z

By looking at the code, I'd rather have it refactored first, then optimize. At least some parts that are crying to be removed, such as readSingleFile -- PTAL #283

dcantah · 2023-04-07T22:50:16Z

@kolyshkin ack, there is quite a few places that all do the same thing. I'm fine getting rid of them and then rebasing here

cgroup2/manager.go

cgroup2/utils_test.go

cgroup2/utils.go

dcantah · 2023-04-24T11:20:01Z

Okay, updated to include the runc trick @kolyshkin, great suggestion. We're down to 199 now.

dcantah · 2023-04-24T11:25:24Z

cgroup2/utils.go

+	for _, pagesize := range hugePageSizes() {
+		usage = append(usage, &stats.HugeTlbStat{
+			Max:      getStatFileContentUint64(filepath.Join(path, "hugetlb."+pagesize+".max")),
+			Current:  getStatFileContentUint64(filepath.Join(path, "hugetlb."+pagesize+".current")),
+			Pagesize: pagesize,
+		})


So while I do love how this looks now, we'd include N entries (where the N is the amount of hugepages we found in /sys/kernel/mm/hugepages) in this slice now whereas before if we failed to find any hugetlb.x.y files we'd return an empty slice. If someone didn't have the hugetlb controller on it may be odd to see multiple entries all with zero stats. There's not a great way to handle this if we use getStatFileContentUint64 as it doesn't error, it just returns 0, which are valid numbers for these. I don't want to make getStatFileContentUint64 error either as it makes the other 7 uses of it more annoying. We could just make an anonymous function here that does what getStatFileContentUint64 does but errors on failures and we'll just continue in the loop, or leave if no one has concerns

Hmm! Can we do soemthing like

hugePageSizeSlice := hugePageSizes() var usage = make(*stats.HugeTlbStat{}, len(hugePageSizeSlice) and then iterate like an array for idx, pageSize := range hugePageSizeSlice { usage[idx] = .... }

This is the same outcome though right? We still have a slice with empty entries, where the number of them is tied to how many huge page sizes we found in /sys/kernel/mm/hugepages. Before this we'd return an empty slice which I think is ideal

Oh okay! yes. what I suggested matches your current code; but I was looking from an allocation perspective. If you pre-allocate; the slice won't have to expand while looping and appending.

I do not have an opinion on functionality as I do not know how users use it. I would try and keep the previous behavior though.

I agree with @manugupt1 that pre-allocating the usage slice is better than growing it dynamically. Even if it won't shave off any allocations, this is more optimized overall, and since we know the size, it makes total sense to do so.

Oh I agree about pre-allocating, more the point I was trying to get across still stands: this behavior is different than whats here today. We'll have entries in hugetlb stats regardless of if someone even has the controller enabled, as long as there's entries in /sys/kernel/mm/hugepages

Right! I am not the best judge of this tbh!

Lets see what others think, I don't think this is too crazy a departure

cgroup2/testutils_test.go

cgroup2/utils.go

manugupt1 · 2023-04-26T06:03:34Z

cgroup2/utils.go

+	for _, pagesize := range hugePageSizes() {
+		usage = append(usage, &stats.HugeTlbStat{
+			Max:      getStatFileContentUint64(filepath.Join(path, "hugetlb."+pagesize+".max")),
+			Current:  getStatFileContentUint64(filepath.Join(path, "hugetlb."+pagesize+".current")),
+			Pagesize: pagesize,
+		})


Hmm! Can we do soemthing like

hugePageSizeSlice := hugePageSizes() var usage = make(*stats.HugeTlbStat{}, len(hugePageSizeSlice) and then iterate like an array for idx, pageSize := range hugePageSizeSlice { usage[idx] = .... }

cgroup2/manager_test.go

This change works towards bringing down allocations for manager.Stat(). There's two things this change does: 1. Swap to opening a file and issuing a single read for uint64/"max" values. Previously we were doing os.ReadFile's which returns a byte slice, so it needs to allocate. 2. Sizing the map we store {controller}.stat values in. We store 40+ stats in this map and the default bucket size for Go seems to be smaller than this, so we'd incur a couple allocs at runtime when adding these. Signed-off-by: Danny Canter <danny@dcantah.dev>

cgroup2/utils.go

kolyshkin

LGTM overall, except for preallocating the usage slice would be better.

@dcantah also, if you like, you can add Co-authored-by: Kir Kolyshkin <kolyshkin@gmail.com> to the second commit, as I wrote that venerable /sys/kernel/mm/hugepages/* parsing code.

kolyshkin · 2023-04-27T02:49:00Z

All in all on Go 1.19.4 this drops allocations in Stat from 322 to 199, or 47%.

Oh, and if you can use benchstat for such calculations that'd be terrific.

dcantah · 2023-04-27T02:59:46Z

@kolyshkin Sure thing! I didn't even realize you wrote that, so you've already gone through this pain 🤣

In the journey of continuing to reduce allocations in manager.Stat, this sets its eyes on optimizing readHugeTlbStats. The number of allocs shaved off here is tied to the number of files in the cgroup directory as f.Readdir(-1) was being called which returns a slice of interfaces. To optimize this we can use a trick runc's cgroup code does which is to take advantage of the fact that we know exactly what files we need to open as they're fixed, the only variable portion is the hugepage sizes available on the host. To get around this we can compute the hugepage sizes in a sync.Once, and all future manager.Stat calls can reap the benefits as we don't need to Readdir() the entire cgroup path to search for hugetlb.pagesize.foobar's. This brings the total number of allocations on Go 1.19.4 down to 199 from the starting point of 322. Signed-off-by: Danny Canter <danny@dcantah.dev> Co-authored-by: Kir Kolyshkin <kolyshkin@gmail.com>

dcantah · 2023-04-28T10:19:46Z

Oh, and if you can use benchstat for such calculations that'd be terrific.

No wonder you recommended, I fat fingered 47 from 38.. No more programming past 12 🐸

fuweid

LGTM

dcantah added the enhancement label Apr 7, 2023

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dcantah changed the title ~~Cgroups2: Reduce allocations for manager.Stat~~ Cgroup2: Reduce allocations for manager.Stat Apr 11, 2023

dcantah marked this pull request as draft April 13, 2023 20:42

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estesp merged commit a6075af into containerd:main May 10, 2023

This was referenced Jun 29, 2023

Reduce allocations for Stat in cg2 #280

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go.mod: Update cgroups to 3.0.2 containerd/containerd#8762

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[release/1.7] go.mod: Update cgroups to 3.0.2 containerd/containerd#8769

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Cgroup2: Reduce allocations for `manager.Stat` #281

Cgroup2: Reduce allocations for `manager.Stat` #281

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Cgroup2: Reduce allocations for manager.Stat #281

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Cgroup2: Reduce allocations for `manager.Stat` #281

Cgroup2: Reduce allocations for `manager.Stat` #281

dcantah commented Apr 7, 2023 •

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