On 2/2/2016 5:34 PM, Benjamin Smith wrote:
I'd like to know what the cause of a particular DB server's slowdown might be.
We've ruled out IOPs for the disks (~ 20%) and raw CPU load (top shows perhaps
1/2 of cores busy, but the system slows to a crawl.
We're suspecting that we're simply running out of memory bandwidth but have no
way to confirm this suspicion. Is there a way to test for this? Think: iostat
but for memory bandwidth instead of disk IO.
memory bandwidth would show up as CPU busy, there's no distinction.
50% of your cores 100% busy, how many cores and how many waiting
database tasks are there? typically with most database servers, one
user connection == one core at a time. so if you have 16 cores, and
only 8 busy/active database connections, that will tie up those 8 cores
and leave the other 8 free. now the 8 processes will probably get
bounced around between the cores, so it could end up looking like all 16
cores are 50% busy averaged over some sample rate, but thats the same
net difference..
--
john r pierce, recycling bits in santa cruz
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