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    SubjectRe: [RFC][PATCH] linux-2.5.34_vsyscall_A0
    Stephen Hemminger wrote:
    > One reason gettimeofday ends up being important is that several
    > databases call it a lot. They use it to build up a transaction id. Under
    > big transaction loads, even the fast linux syscall path ends up being a
    > bottleneck. Also, on NUMA machines the data used for time of day (xtime)
    > ends up being a significant portion of the cache traffic.
    > It would be great to rework the whole TSC time of day stuff to work with
    > per cpu data and allow unsychronized TSC's like NUMA. The problem is
    > that for fast user level access, there would need to be some way to find
    > out the current CPU and avoid preemption/migration for a short period.
    > It seems like the LDT stuff for per-thread data could provide the
    > current cpu (and maybe current pid) somehow. And it would be possible
    > to avoid preemption while in a vsyscall text page, some other Unix
    > variants do this to implement portions of the thread library in kernel
    > provided user text pages.
    Now there is an idea! Lock preemption in user space if and
    only if the user is executing in a text page shared with the
    kernel. I have seen the need for such locking, but have
    always thought they were too dangerous. This convention
    would introduce a much higher level of security. What is
    left is to devise a way to let the kernel know that the
    preemption targeted task has left such a page so that the
    preemption may proceed. Possibly the kernel could plant a
    hint on the page that each function would check for on exit.
    George Anzinger
    Preemption patch:
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