CIO/DIO and JFS2 read ahead


 
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Operating Systems AIX CIO/DIO and JFS2 read ahead
# 8  
Old 11-14-2008
pstat -a shows, like for example ps aux, only the number of current started aioservers, but not if they are busy or not. With nmon shift+a you see how many are really currently busy and this is on system with an app using AIO, going up and down. Often the maxreqs is forgotten to be configured somewhat large too so the app doesn't get errors regarding to hit the max of queued AIO requests (some "error 5" in Oracle for example).

Nevertheless, we will have to check out if the OP needs AIO at all, which he could and should find out. Maybe he uses it already, no idea. Maybe he offers us some more info to help him.
In our environments I used CIO as a kind of last ressort of tuning, when everything else with AIO and ioo tuning didn't help anymore.
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drv_usecwait(9F)					   Kernel Functions for Drivers 					  drv_usecwait(9F)

NAME
drv_usecwait - busy-wait for specified interval SYNOPSIS
#include <sys/types.h> #include <sys/ddi.h> void drv_usecwait(clock_t microsecs); INTERFACE LEVEL
Architecture independent level 1 (DDI/DKI). PARAMETERS
microsecs The number of microseconds to busy-wait. DESCRIPTION
drv_usecwait() gives drivers a means of busy-waiting for a specified microsecond count. The amount of time spent busy-waiting may be greater than the microsecond count but will minimally be the number of microseconds specified. delay(9F) can be used by a driver to delay for a specified number of system ticks, but it has two limitations. First, the granularity of the wait time is limited to one clock tick, which may be more time than is needed for the delay. Second, delay(9F) may only be invoked from user context and hence cannot be used at interrupt time or system initialization. Often, drivers need to delay for only a few microseconds, waiting for a write to a device register to be picked up by the device. In this case, even in user context, delay(9F) produces too long a wait period. CONTEXT
drv_usecwait() can be called from user or interrupt context. SEE ALSO
delay(9F), timeout(9F), untimeout(9F) Writing Device Drivers NOTES
The driver wastes processor time by making this call since drv_usecwait() does not block but simply busy-waits. The driver should only make calls to drv_usecwait() as needed, and only for as much time as needed. drv_usecwait() does not mask out interrupts. SunOS 5.10 12 Nov 1992 drv_usecwait(9F)