Is there a memory leak in bigcache?

[Kaushal28]

I initially set a 10k keys and then for multiple times, I cleared half of them and set them again. In total the number of keys are constant (10k) all the time and all the key names and values are exactly the same. I just Delete() and Set() the same keys multiple times on the same bigcache instance.

When I did this 5 times, the memory profile (pprof) showed total of ~396MB memory being allocated at the end. But When I did the same 25 time, it showed ~2.26GB memory still allocated. Since I am clearing and then re-setting the key value pairs, shouldn't the memory utilization be same irrespective of number of time I do it as long as the key-value pair content and number of key-values are exactly same?

Why memory utilization increases as I do more Delete and Set?

Here is the script that I used to do it for better understanding and reproducibility:

import (
    "fmt"
    "strings"
    "time"

    "net/http"
    _ "net/http/pprof"

    "github.com/allegro/bigcache/v2"
)

const (
    count = 10000
)

// Just a helper to get a large value string/chunk.
func gen_chunk() []byte {
    var arr []string
    for i := 0; i < 1000; i++ {
        arr = append(arr, fmt.Sprintf("Test Value %d", i))
    }
    return []byte(strings.Join(arr, ", "))
}

func main() {
    cache, err := bigcache.NewBigCache(bigcache.DefaultConfig(7 * time.Hour))
    if err != nil {
        fmt.Printf("Error: %v\n", err.Error())
    }
    chunk := gen_chunk()
    go func() {
        // Set 10k keys with large values
        for i := 0; i < count; i++ {
            cache.Set(fmt.Sprintf("my-unique-key-%d", i), chunk)
        }

        for j := 0; j < 25; j++ {
            // Remove half of the keys
            for i := 0; i < count/2; i++ {
                cache.Delete(fmt.Sprintf("my-unique-key-%d", i))
            }

            // Again add those keys
            for i := 0; i < count/2; i++ {
                cache.Set(fmt.Sprintf("my-unique-key-%d", i), chunk)
            }
            // Empty all the cache
            // cache.Reset().     // Just FYI: When I do this after every iteration, the memory utilization stays constant.
            fmt.Printf("###: Itr: %d\n", j)
        }
        fmt.Println("### Done!")
    }()
    http.ListenAndServe("localhost:6060", nil)
}

Note that I am using github.com/allegro/bigcache/v2 v2.2.5 and this profiling is from http://localhost:6060/debug/pprof/heap

Here is the mem profile result when I did it 25 times:

Showing nodes accounting for 2.30GB, 99.89% of 2.30GB total
Dropped 19 nodes (cum <= 0.01GB)
Showing top 10 nodes out of 11
      flat  flat%   sum%        cum   cum%
    2.26GB 98.08% 98.08%     2.26GB 98.08%  github.com/allegro/bigcache/v2/queue.(*BytesQueue).allocateAdditionalMemory
    0.03GB  1.12% 99.20%     0.03GB  1.14%  github.com/allegro/bigcache/v2.initNewShard
    0.02GB  0.69% 99.89%     0.02GB  0.69%  github.com/allegro/bigcache/v2.wrapEntry (inline)
         0     0% 99.89%     2.27GB 98.77%  github.com/allegro/bigcache/v2.(*BigCache).Set
         0     0% 99.89%     2.27GB 98.77%  github.com/allegro/bigcache/v2.(*cacheShard).set
         0     0% 99.89%     0.03GB  1.14%  github.com/allegro/bigcache/v2.NewBigCache (inline)
         0     0% 99.89%     0.03GB  1.14%  github.com/allegro/bigcache/v2.newBigCache
         0     0% 99.89%     2.26GB 98.08%  github.com/allegro/bigcache/v2/queue.(*BytesQueue).Push
         0     0% 99.89%     0.03GB  1.14%  main.main
         0     0% 99.89%     2.27GB 98.77%  main.main.func1

Here is the mem profile result when I did it 5 times:

  393.97MB 72.72% 72.72%   393.97MB 72.72%  github.com/allegro/bigcache/v2/queue.(*BytesQueue).allocateAdditionalMemory
   93.98MB 17.35% 90.07%    93.98MB 17.35%  github.com/allegro/bigcache/v2/queue.NewBytesQueue (inline)
   31.99MB  5.90% 95.98%   125.97MB 23.25%  github.com/allegro/bigcache/v2.initNewShard
   19.30MB  3.56% 99.54%    19.30MB  3.56%  github.com/allegro/bigcache/v2.wrapEntry (inline)
         0     0% 99.54%   413.27MB 76.28%  github.com/allegro/bigcache/v2.(*BigCache).Set
         0     0% 99.54%   413.27MB 76.28%  github.com/allegro/bigcache/v2.(*cacheShard).set
         0     0% 99.54%   125.97MB 23.25%  github.com/allegro/bigcache/v2.NewBigCache (inline)
         0     0% 99.54%   125.97MB 23.25%  github.com/allegro/bigcache/v2.newBigCache
         0     0% 99.54%   393.97MB 72.72%  github.com/allegro/bigcache/v2/queue.(*BytesQueue).Push
         0     0% 99.54%   125.97MB 23.25%  main.main

Notice the difference in the memory utilization above. Is that a sign on memory leak? Please help.

[janisz]

You need to specify HardMaxCacheSize in order to keep memory limited. Without it cache will only grow.

https://github.com/allegro/bigcache/blob/caa34156bfeab3aa547e18c21292736bbd2ae2c2/config.go#L25-L32

https://github.com/allegro/bigcache/blob/caa34156bfeab3aa547e18c21292736bbd2ae2c2/config.go#L57-L67

[Kaushal28]

@janisz, but why cache will only grow even if the number of key/value pairs is constant all the time and the content (both key and values) that I am setting again after removing is also exactly the same? I understand that there might be some additional space requirements by library itself for internal handling (overhead as mentioned in the comments of HardMaxCacheSize above) but shouldn't it remain almost constant if the key values are the same overtime? Why am I seeing GBs of fluctuations as I do more and more Set() and Removes()?

[Kaushal28]

I also tried the same above script with various other Go in-memory caching libraries like go-cache, ttl-cache and even the native Go maps. Non of them are showing this much fluctuation. Also, the total memory footprint is also way higher in bigcache. So I was wondering if there is any specific reason for that?

[Kaushal28]

I could find many other similar open issues/bug stating almost the same problem as I mentioned:

https://github.com/allegro/bigcache/issues/311 https://github.com/allegro/bigcache/issues/353 https://github.com/allegro/bigcache/issues/309 https://github.com/allegro/bigcache/issues/109

From all these, it seems that when over writing existing key with same value or deleting keys would not actually delete the existing value and will keep adding new content to the cache and it's intentional. So we must set max hard limit to cache size HardMaxCacheSize. Is it right? If what I mentioned is correct and that's intentional, may I know the exact reason behind this design choice? Thanks.

@janisz