runtime: clarify GC fractional mode description
nowdays, in runtime/mgc.go,we can see the comment descrition : The fractional worker is necessary when GOMAXPROCS*gcBackgroundUtilization is not an integer.
but it not true such as GOMAXPROCS=5.
in the implemet of startCycle() , Fractional Mode happend only when
GOMAXPROCS<=3 or GOMAXPROCS=6. so utilization can closest to 25%.
Fixes #44380
Change-Id: Id0dd6d9f37759c2c9231f164a013a014216dd442
GitHub-Last-Rev:
5910e76324b2fa908235c325c8b1edafca496256
GitHub-Pull-Request: golang/go#44381
Reviewed-on: https://go-review.googlesource.com/c/go/+/293630
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Pratt <mpratt@google.com>