Messages in this thread | | | Date | Tue, 22 Oct 2002 22:03:15 -0400 | From | Jeff Garzik <> | Subject | Re: [PATCH] linux-2.5.44_vsyscall_A1 |
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john stultz wrote: > All, > Here is the next rev of the i386 vsyscall gettimeofday port I sent > earlier. I've added a fix for the linker trouble in the last one, as > well as wrapped the whole thing in CONFIG_VSYSCALL. > > This patch implements gettimeofday in a user readable page, allowing > for calls to gettimeofday to execute completely in userspace, giving a > significant performance boost. > > Changes to glibc are unnecessary, because users that want to use the > vsyscall can do so by LD_PRELOADING a library which alias gettimeofday > before executing their application. This will not affect any other > application and allows the backward compatibility issue to be ignored. > I've created an example library (to be attached in a following email) > and ran a quick performance test w/ and w/o the preloaded library, > giving the following results: > > Normal gettimeofday > gettimeofday ( 1403307us / 1000000runs ) = 1.403306us > vsyscall LD_PRELOAD gettimeofday > gettimeofday ( 285423us / 1000000runs ) = 0.285423us > > Since this code uses the TSC for calculating time of day, this patch > will not help systems that suffer from TSC skew (ie: many NUMA systems, > etc). However, for UP and SMP boxes this is a pretty major win. > Alternative methods to use the cyclone/HPET registers for NUMA boxes are > also feasible in the future. > > Please let me know your thoughts about 2.5 integration.
In terms of implementation, I think it's way too x86-specific... some of the vsyscall infrastructure can be more generic, making it easier for other arches to implement the same functionality. Also use of TSC isn't a terribly good idea...
Overall the idea has existed for a while, and I think it's a good one... gettimeofday(2) is very definitely a performance bottleneck in databases, in apache [if nothing's changed in that area...], etc.
Jeff
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