Latch, mutex and beyond

February 23, 2011

Latch statistics

Filed under: DTrace,Instrumentation,Latch,Latch Statistics,Theory,Uncategorized — andreynikolaev @ 10:36 am

In previous posts, I investigated how the Oracle process spins and waits for the latch. Now we need the tool to estimate when the latch acquisition works efficiently and when we need to tune it. This tool is the latch statistics. Contemporary Oracle documentation describes v$latch statistics columns as:

Statistic: x$ksllt column Documentation description: When and how it changed:
GETS kslltwgt
“wait gets”
Number of times the latch was requested in willing-to-wait mode Incremented by one after latch acquisition. Therefore protected by latch
MISSES kslltwff
“wait fails”
Number of times the latch was requested in willing-to-wait mode and the requestor had to wait Incremented by one after latch acquisition if miss occured
SLEEPS kslltwsl
“wait sleeps”
Number of times a willing-to-wait latch request resulted in a session sleeping while waiting for the latch Incremented by number of times process slept during latch acquisition
SPIN_GETS ksllthst0 Willing-to-wait latch requests which missed the first try but succeeded while spinning Incremented by one after latch acquisition if miss but not sleep occured. Counts only the first spin
WAIT_TIME kslltwtt
“wait time”
Elapsed time spent waiting for the latch (in microseconds) Incremented by wait time spent during latch acquisition.
IMMEDIATE_GETS kslltngt
“nowait gets”
Number of times a latch was requested in no-wait mode Incremented by one after each no-wait latch get. May not be protected by latch
IMMEDIATE_MISSES kslltnfa
“nowait fails”
Number of times a no-wait latch request did not succeed Incremented by one after unsuccessful no-wait latch get. Not protected by latch

(more…)

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