* Andrew Morton <[email protected]> wrote:
> > We are pleased to announce the first release of the "lock dependency
> > correctness validator" kernel debugging feature
>
> What are the runtime speed and space costs of enabling this?
The RAM space costs are estimated in the bootup info printout:
... MAX_LOCKDEP_SUBTYPES: 8
... MAX_LOCK_DEPTH: 30
... MAX_LOCKDEP_KEYS: 2048
... TYPEHASH_SIZE: 1024
... MAX_LOCKDEP_ENTRIES: 8192
... MAX_LOCKDEP_CHAINS: 8192
... CHAINHASH_SIZE: 4096
memory used by lock dependency info: 696 kB
per task-struct memory footprint: 1200 bytes
Plus every lock now embedds the lock_map structure which is 10 pointers.
That is the biggest direct dynamic RAM cost.
There are also a few embedded keys in .data but they are small.
The .text overhead mostly comes from the subsystem itself - which is
around 20K of .text. The callbacks are not inlined most of the time -
there are about 200 of them right now, which should be another +1-2K of
.text cost.
The runtime cycle cost is significant if CONFIG_DEBUG_LOCKDEP [lock
validator self-consistency checks] is enabled - then we take a global
lock from every lock operation which kills scalability.
If DEBUG_LOCKDEP is disabled then it's OK - smaller than DEBUG_SLAB. In
this case we have the lock-stack maintainance overhead, the irq-trace
callbacks and a lockless hash-lookup per lock operation. All of that
overhead is O(1) and lockless so it shouldnt change fundamental
characteristics anywhere.
Ingo
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