recommendations for max_servers

[Louis Munro]

Hello,

While troubleshooting a system I came upon a case of 'hung children' and duplicate requests. 
I would usually ascribe this to a database issue but in this case the database is mostly unused and properly indexed. 
Accounting is not used, so that's one less thing to consider.

On the other hand the max_servers setting had been set as high as 192 by someone with good intentions. 
Tuning it down to 64 seemed to significantly reduce the load on the system and the number of hung children was reduced by a factor of about 100. 

While there remains an issue with some (intermittent) slow ntlm_auth to take care of, I wondered how others tune the value of max_servers other than by trial and error. Most of the time the default of 32 has been enough for me. Higher is not necessarily better in my experience since at least in this case it seems to have led to the main thread working harder when under load (with most of the work done in the "system" space). 

This is a system running 2.2.5 on RHEL 6.4 in VmWare. It's got 24Gb of RAM and 16 cores so it should still be pretty capable. 

Does anyone have an algorithm, rule of thumb or other ballpark way of estimating the "ideal" maximum number of threads? 

Regards,
--
Louis Munro
lmunro at inverse.ca  ::  www.inverse.ca 
+1.514.447.4918 x125  :: +1 (866) 353-6153 x125
Inverse inc. :: Leaders behind SOGo (www.sogo.nu) and PacketFence (www.packetfence.org)

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