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>> I have these questions: >> >> * There is a rw semaphore that is locked for read for nearly all operations >> and locked for write only rarely. However locking for read causes cache >> line pingpong on SMP systems. Do you have an idea how to make it better? >> >> It could be improved by making a semaphore for each CPU and locking for >> read only the CPU's semaphore and for write all semaphores. Or is there a >> better method? >> > > If you believe you need a semaphore for protecting a mostly read structure, > then RCU is certainly a good candidate. (ie no locked operation at all) RCU is for non-blocking operation only. Maybe it could be use for maintaining an unlocked variable that prevents readers from entering a critical section --- but waiting in a write for a quiscent state would hurt. > The problem with a per_cpu biglock is that you may consume a lot of RAM for > big NR_CPUS. Count 32 KB per 'biglock' if NR_CPUS=1024 > >> * This leads to another observation --- on i386 locking a semaphore is 2 >> instructions, on x86_64 it is a call to two nested functions. Has it some >> reason or was it just implementator's laziness? Given the fact that locked >> instruction takes 16 ticks on Opteron (and can overlap about 2 ticks with >> other instructions), it would make sense to have optimized semaphores too. > > Hum, please dont use *lazy*, this could make Andi unhappy :) > > What are you calling semaphore exactly ? > Did you read Documentation/mutex-design.txt ? I see --- I could replace that semaphore with mutex. But rw-semaphores can't be replaces with them. Mikulas - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | |||||||||
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