Messages in this thread |  | | | Date | Tue, 03 Nov 2009 07:12:02 +0200 | | From | Avi Kivity <> | | Subject | Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation |
| |
On 11/02/2009 05:46 PM, Dan Magenheimer wrote: >>> I don't have any public data available for this DB usage, >>> >> Sorry, that doesn't explain anything. >> > Well for now just consider the DB usage as another use > of profiling. But one can easily draw scenarios where > a monotonic timestamp is also used to guarantee transaction > ordering. >
In this case we should provide a facility for this. Providing a global monotonic counter may be easier than providing a monotonic clock. Hence my question.
>>> Search for "flight recorder". This feature is intended to >>> be enabled all the time, but with non-vsyscall gettimeofday >>> the performance impact is unacceptably high, so they are using >>> >> For profiling work fast timestamping is of course great, but surely >> there is no monotonicity requirement? >> > Yes and no. Monotonicity is a poor substitute for a more > generic mechanism that might provide an indication that a > discontinuity has occurred (forward or backward); if an app > could get both the timestamp AND some kind of "continuity > generation counter" (basically a much more sophisticated > form of TSC_AUX that changes whenever the timestamp is > coming from a different source), perhaps all problems could be solved. >
I doubt it. A discontinuity has occured, but what do we know about it? nothing.
>> I don't think we'll be able to provide monotonicity with vsyscall on >> tsc-broken hosts, so we'll be limited to correcting the tsc frequency >> after migration for good-tsc hosts. >> > True, though clock_gettime(CLOCK_MONOTONIC) can provide > the monotonicity where it is required. >
We have that already. The question is how to implement it in a vsyscall.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
|  |