CO-RE pragma add lots of memory overhead about ebpf

[lmb]

What you are seeing is an interaction with the Go GC. Flushing the caches doesn't automatically trigger GC. In the XDP example, no more GCs are triggered while in a "normal" application you will be allocating things, so sooner or later the memory should be returned. If you don't allocate you need to run runtime.GC() manually after flushing the caches:

package main

import (
	"runtime/debug"
	"testing"
	"time"

	"github.com/cilium/ebpf/linux"
)

func TestLoad(t *testing.T) {
	_, _ = linux.Types()

	linux.FlushCaches()

	runtime.GC()

	time.Sleep(time.Minute)
}

Execute that using GODEBUG=gctrace=1 and you'll see something like:

$ go test -c ./examples/xdp/ && GODEBUG=gctrace=1 ./xdp.test 
gc 1 @0.002s 2%: 0.013+1.1+0.009 ms clock, 0.20+0.049/1.1/0+0.15 ms cpu, 3->4->3 MB, 4 MB goal, 0 MB stacks, 0 MB globals, 16 P
gc 2 @0.007s 3%: 0.010+1.5+0.009 ms clock, 0.17+0.060/2.9/0+0.14 ms cpu, 9->10->9 MB, 9 MB goal, 0 MB stacks, 0 MB globals, 16 P
gc 3 @0.034s 1%: 0.013+2.5+0.008 ms clock, 0.21+0.33/5.4/0.32+0.13 ms cpu, 19->20->15 MB, 20 MB goal, 0 MB stacks, 0 MB globals, 16 P
gc 4 @0.059s 2%: 0.050+11+0.004 ms clock, 0.80+0.44/12/0.005+0.073 ms cpu, 33->33->24 MB, 33 MB goal, 0 MB stacks, 0 MB globals, 16 P
gc 5 @0.102s 2%: 0.019+6.2+0.011 ms clock, 0.31+0.051/20/34+0.18 ms cpu, 46->48->47 MB, 49 MB goal, 0 MB stacks, 0 MB globals, 16 P
gc 6 @0.138s 2%: 0.019+0.24+0.002 ms clock, 0.31+0/0.16/0+0.042 ms cpu, 65->65->0 MB, 94 MB goal, 0 MB stacks, 0 MB globals, 16 P (forced)
PASS

The last line is where the memory held by BTF is released. https://www.ardanlabs.com/blog/2019/05/garbage-collection-in-go-part2-gctraces.html has a good explanation for what the debug output means.

Still, this doesn't release memory to the OS immediately! Seems like the GC has a heuristic for when to return memory. You can short circuit this by replacing runtime.GC() with debug.FreeOSMemory(). After doing this I see RES return to normal-ish levels.

My recommendation: if your application doesn't allocate at all after attaching the BPF, add a runtime.GC call after FlushCaches. Otherwise let the GC do its thing. I'd be wary of adding debug.FreeOSMemory to a production app.

[brycekahle]

Have you collected a profile using pprof to see if there is a change in Go allocations?

[msherif1234]

Have you collected a profile using pprof to see if there is a change in Go allocations?

they look identical from go allocations perspective

• run with CO-RE

(pprof) top10
Showing nodes accounting for 1024.70kB, 100% of 1024.70kB total
Showing top 10 nodes out of 13
      flat  flat%   sum%        cum   cum%
  512.50kB 50.01% 50.01%   512.50kB 50.01%  runtime.allocm
  512.20kB 49.99%   100%   512.20kB 49.99%  runtime.malg
         0     0%   100%   512.50kB 50.01%  runtime.mstart
         0     0%   100%   512.50kB 50.01%  runtime.mstart0
         0     0%   100%   512.50kB 50.01%  runtime.mstart1
         0     0%   100%   512.50kB 50.01%  runtime.newm
         0     0%   100%   512.20kB 49.99%  runtime.newproc.func1
         0     0%   100%   512.20kB 49.99%  runtime.newproc1
         0     0%   100%   512.50kB 50.01%  runtime.resetspinning
         0     0%   100%   512.50kB 50.01%  runtime.schedule

• run with CO-RE disabled

(pprof) top10
Showing nodes accounting for 1024.70kB, 100% of 1024.70kB total
Showing top 10 nodes out of 13
      flat  flat%   sum%        cum   cum%
  512.50kB 50.01% 50.01%   512.50kB 50.01%  runtime.allocm
  512.20kB 49.99%   100%   512.20kB 49.99%  runtime.malg
         0     0%   100%   512.50kB 50.01%  runtime.mstart
         0     0%   100%   512.50kB 50.01%  runtime.mstart0
         0     0%   100%   512.50kB 50.01%  runtime.mstart1
         0     0%   100%   512.50kB 50.01%  runtime.newm
         0     0%   100%   512.20kB 49.99%  runtime.newproc.func1
         0     0%   100%   512.20kB 49.99%  runtime.newproc1
         0     0%   100%   512.50kB 50.01%  runtime.resetspinning
         0     0%   100%   512.50kB 50.01%  runtime.schedule

[msherif1234]

cc @lmb

[lmb]

I extracted the loadBpf call into a test and collected a memory profile.

(pprof) top     
Showing nodes accounting for 45474.43kB, 100% of 45474.43kB total
Showing top 10 nodes out of 30
      flat  flat%   sum%        cum   cum%
14768.18kB 32.48% 32.48% 14768.18kB 32.48%  github.com/cilium/ebpf/btf.indexTypes
13825.40kB 30.40% 62.88% 15948.08kB 35.07%  github.com/cilium/ebpf/btf.inflateRawTypes
 6329.13kB 13.92% 76.80% 10937.92kB 24.05%  github.com/cilium/ebpf/btf.readTypes
 4096.79kB  9.01% 85.81%  4608.80kB 10.13%  encoding/binary.Read
 2283.86kB  5.02% 90.83%  3307.88kB  7.27%  github.com/cilium/ebpf/btf.readStringTable
 2122.68kB  4.67% 95.50%  2122.68kB  4.67%  github.com/cilium/ebpf/btf.inflateRawTypes.func3

The memory increase you're seeing is triggered by CO-RE, but the CO-RE code itself doesn't allocate a lot. To do CO-RE relocations we need to parse the vmlinux BTF. Parsing that is expensive and the parsed representation uses a bunch of resident memory.

If only ever load BPF at the start of your program you can use https://pkg.go.dev/github.com/cilium/ebpf/btf#FlushKernelSpec to release the memory used by the cached kernel BTF.

[msherif1234]

at what point just after the load I can try that in the xdp example in my WIP pr and rerun ?

[lmb]

After loadBpf() returns.

[msherif1234]

so I applied the following changes to xdp test program

diff --git a/examples/xdp/main.go b/examples/xdp/main.go
index a669a86..67fc76d 100644
--- a/examples/xdp/main.go
+++ b/examples/xdp/main.go
@@ -17,6 +17,7 @@ import (
        "time"
 
        "github.com/cilium/ebpf"
+       "github.com/cilium/ebpf/btf"
        "github.com/cilium/ebpf/link"
 )
 
@@ -41,7 +42,7 @@ func main() {
                log.Fatalf("loading objects: %s", err)
        }
        defer objs.Close()
-
+       btf.FlushKernelSpec()
        // Attach the program.
        l, err := link.AttachXDP(link.XDPOptions{
                Program:   objs.XdpProgFunc,

however memory seems to be getting worse than before ?

Note: repro changes are here https://github.com/msherif1234/ebpf/tree/test_fix_core if u would like to see the same

[msherif1234]

my application dynamically allocate memories because I am using BPF_F_NO_PREALLOC so I don't think I can use runtime.GC() correct ?

[lmb]

BPF memory and Go memory are completely distinct. runtime.GC has no effect on BPF memory.