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vmselect: ensure default -search.maxConcurrentRequests is non-decreasing (#6996)

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### Describe Your Changes

vmselect determines the default value of `-search.maxConcurrentRequests`
multiplying the number of available CPUs by 2 if and only if the number
is small (to be precise <= 4). That leads
`-search.maxConcurrentRequests` is decreasing at the edge of these two
cases as shown below:
| CPUs | MaxConcurrentRequests | MaxConcurrentRequests (original
proposal) | MaxConcurrentRequests (updated proposal) |
|--------|--------|--------|--------|
| 1 | 2 | 2 | 2 |
| 2 | 4 (prev+2) | 4 (prev+2) | 4 (prev+2) |
| 3 | 6 (prev+2) | 6 (prev+2) | 6 (prev+2) |
| 4 | 8 (prev+2) | 8 (prev+2) | 8 (prev+2) |
| 5 | 5 __(prev-3)__ | 9 __(prev+1)__ | 10 __(prev+2)__ |
| 6 | 6 (prev+1) | 10 (prev+1) | 12 (prev+2) |
| 7 | 7 (prev+1) | 11 (prev+1) | 14 (prev+2) |
| 8 | 8 (prev+1) | 12 (prev+1) | 16 (prev+2) |

I propose to make the default value non-decreasing.
This commit is contained in:
Alexander Frolov 2024-09-30 09:51:54 +02:00 committed by GitHub
parent 2df2e9b92f
commit 80a3c410d4
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1 changed files with 1 additions and 4 deletions
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5
app/vmselect/main.go
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@ -44,10 +44,7 @@ var (
var slowQueries = metrics.NewCounter(`vm_slow_queries_total`)
func getDefaultMaxConcurrentRequests() int {
n := cgroup.AvailableCPUs()
if n <= 4 {
n *= 2
}
n := cgroup.AvailableCPUs() * 2
if n > 16 {
// A single request can saturate all the CPU cores, so there is no sense
// in allowing higher number of concurrent requests - they will just contend