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    SubjectRe: [RFC&PATCH] Alternative RCU implementation
    On Mon, 2004-08-30 at 14:52, Paul E. McKenney wrote: 
    > On Mon, Aug 30, 2004 at 01:13:41PM -0400, Jim Houston wrote:
    > > I know that I'm questioning one of your design goals for RCU by adding
    > > overhead to the read-side. I have read everything I could find on RCU.
    > > My belief is that the cost of the xchg() instruction is small
    > > compared to the cache benifit of freeing memory more quickly.
    > > I think it's more interesting to look at the impact of the xchg() at the
    > > level of an entire system call. Adding 30 nanoseconds to a open/close
    > > path that tasks 3 microseconds seems reasonable. It is hard to measure
    > > the benefit of reusing the a dcache entry more quickly.
    > Hello, Jim,
    > The other thing to keep in mind is that reducing the grace-period
    > duration increases the per-access overhead, since each grace period
    > incurs a cost. So there is a balance that needs to be struck between
    > overflowing memory with a too-long grace period and incurring too
    > much overhead with a too-short grace period.
    > How does the rest of the kernel work with all interrupts to
    > a particular CPU shut off? For example, how do you timeslice?
    > Thanx, Paul
    > PS. My concerns with some aspects of your design aside, your
    > getting a significant change to the RCU infrastructure to
    > work reasonably well is quite impressive!

    Hi Paul,

    I have two module parameters in the patch which can be used to
    tune how often grace periods are started. They can be set at boot
    time as follows:

    The per-cpu count of queued requests at which to
    start a new batch. Patch defaults to 256.

    Timeout value in jiffies at which to start a batch.
    Defaults to HZ/10.

    I picked the defaults to start batches with similar frequency to
    the existing code.

    I tested a dual processor with rcupdate.rcu_max_count=0. This
    will start a grace period for every call_rcu(). I ran
    my rename test this way and it worked suprisingly well.

    I maintain a nxtbatch value which lets me check if the grace period
    for the entries in the nxt list has started or perhaps already
    completed. I check this in call_rcu() and avoid mixing batches.
    Any requests queued before the batch was started will be completed
    at the end of the grace period. Unless a very small rcu_max_count
    value is used, there is likely to be some delay between completing
    a grace period and needing to start another.

    > How does the rest of the kernel work with all interrupts to
    > a particular CPU shut off? For example, how do you timeslice?

    It's a balancing act. In some cases we just document the
    missing functionality. If the local timer is disabled on a cpu,
    all processes are SCHED_FIFO. In the case of Posix timers, we
    move timers to honor the procesor shielding an the process affinity.

    Jim Houston

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