Messages in this thread | | | Date | Mon, 9 Feb 2009 14:15:26 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) |
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* Mathieu Desnoyers (compudj@krystal.dyndns.org) wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > On Mon, Feb 09, 2009 at 10:37:42AM -0800, Paul E. McKenney wrote: > > > On Mon, Feb 09, 2009 at 01:13:41PM -0500, Mathieu Desnoyers wrote: > > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > [ . . . ] > > > > > > You know what ? Changing RCU_GP_CTR_BIT to 16 uses a > > > > testw %ax, %ax instead of a testb %al, %al. The trick here is that > > > > RCU_GP_CTR_BIT must be a multiple of 8 so we can use a full 8-bits, > > > > 16-bits or 32-bits bitmask for the lower order bits. > > > > > > > > On 64-bits, using a RCU_GP_CTR_BIT of 32 is also ok. It uses a testl. > > > > > > > > To provide 32-bits compability and allow the deepest nesting possible, I > > > > think it makes sense to use > > > > > > > > /* Use the amount of bits equal to half of the architecture long size */ > > > > #define RCU_GP_CTR_BIT (sizeof(long) << 2) > > > > > > You lost me on this one: > > > > > > sizeof(long) << 2 = 0x10 > > > > > > I could believe the following (run on a 32-bit machine): > > > > > > 1 << (sizeof(long) * 8 - 1) = 0x80000000 > > > > > > Or, if you were wanting to use a bit halfway up the word, perhaps this: > > > > > > 1 << (sizeof(long) * 4 - 1) = 0x8000 > > > > > > Or am I confused? > > > > Well, I am at least partly confused. You were wanting a low-order bit, > > so you want to lose the "- 1" above. Here are some of the possibilities: > > > > sizeof(long) = 0x4 > > sizeof(long) << 2 = 0x10 > > 1 << (sizeof(long) * 8 - 1) = 0x80000000 > > 1 << (sizeof(long) * 4) = 0x10000 > > 1 << (sizeof(long) * 4 - 1) = 0x8000 > > 1 << (sizeof(long) * 2) = 0x100 > > 1 << (sizeof(long) * 2 - 1) = 0x80 > > > > My guess is that 1 << (sizeof(long) * 4) and 1 << (sizeof(long) * 2) > > are of the most interest. > > > > Exactly. I'll change it to : > > #define RCU_GP_CTR_BIT (1 << (sizeof(long) << 2)) > > I somehow thought this define was used as a bit number rather than the > bit mask. > > Thanks, > > Mathieu >
It's pushed in the git tree. I also removed an increment in the fast path by initializing urcu_gp_ctr to RCU_GP_COUNT.
It brings benchmarks to :
Time per read : 6.87183 to 7.25318 cycles
So we seem to save about half a cycle to a cycle with this.
Mathieu
> > > > Thanx, Paul > > > > -- > Mathieu Desnoyers > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 > > _______________________________________________ > ltt-dev mailing list > ltt-dev@lists.casi.polymtl.ca > http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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