Re: Spinlocks, yet again: analysis and proposed patches

Added to TODO list.

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Simon Riggs wrote:
> On Wed, 2005-09-14 at 13:32 -0400, Tom Lane wrote:
> > I wrote:
> > > Another thought came to mind: maybe the current data layout for LWLocks
> > > is bad. Right now, the spinlock that protects each LWLock data struct
> > > is itself part of the struct, and since the structs aren't large (circa
> > > 20 bytes), the whole thing is usually all in the same cache line. ...
> > > Maybe it'd be better to allocate the spinlocks off by themselves.
> >
> > Well, this is odd. I made up a patch to do this (attached) and found
> > that it pretty much sucks. Still the 4-way Opteron, previous best
> > (slock-no-cmpb and spin-delay-2):
> > 1 31s 2 42s 4 51s 8 100s
> > with lwlock-separate added:
> > 1 31s 2 52s 4 106s 8 213s
> >
> > What I had expected to see was a penalty in the single-thread case (due
> > to instructions added to the LWLock functions), but there isn't any.
> > I did not expect to see a factor-of-2 penalty for four threads.
> >
> > I guess what this means is that there's no real problem with losing the
> > cache line while manipulating the LWLock, which is what the patch was
> > intended to prevent. Instead, we're paying for swapping two cache lines
> > (the spinlock and the LWLock) across processors instead of just one line.
> > But that should at worst be a 2x inflation of the time previously spent
> > in LWLockAcquire/Release, which is surely not yet all of the application
> > ;-). Why the heck is this so bad?
>
> (Just going back through the whole thread for completeness.)
>
> > Should we expect that apparently
> > minor changes in shared data structures might be costing equivalently
> > huge penalties in SMP performance elsewhere?
>
> Yes. That's the advice from Intel and AMD; but we should add that there
> is potential for improving performance also.
>
> The possible problem we were trying to avoid here was false sharing of
> the cache line by lock requestors of locks whose spin locks were
> adjacent in memory.
>
> Splitting the data structure was just one of the possible ways of
> avoiding that. The above shows that the possible solution described
> above didn't work, but there could be others.
>
> One thing we tried in February was padding out the statically defined
> locks with dummy lock definitions in the enum. This has the effect of
> ensuring that the most contested locks are very definitely in their own
> cache line and not shared with others.
> That showed a noticeable improvement in performance, probably because it
> costs very little to implement, even if the code would require some
> explanatory documentation.
>
> The lwlock structure in include/storage/lwlock.h looks like
>
> typedef enum LWLockId
> {
> BufMappingLock,
> BufFreelistLock,
> LockMgrLock,
> OidGenLock,
> XidGenLock,
> ProcArrayLock,
> SInvalLock,
> FreeSpaceLock,
> WALInsertLock,
> WALWriteLock,
> ...
>
> Notice that the heavily contested locks (i.e. first 3 and the WAL locks)
> are adjacent to at least one other heavily contested lock. So they are
> certain to be in the same cache line and therefore to cause false
> sharing (on all CPU types, not just Intel and AMD (ref: Manufacturer's
> optimization handbooks).
>
> This could be replaced by...
>
> typedef enum LWLockId
> {
> BufMappingLock,
> BufMappingLock_Padding1,
> BufMappingLock_Padding2,
> BufMappingLock_Padding3,
> BufMappingLock_Padding4,
> BufMappingLock_Padding5,
> BufMappingLock_Padding6,
> BufMappingLock_Padding7,
> BufFreelistLock,
> BufFreelistLock_Padding1,
> BufFreelistLock_Padding2,
> BufFreelistLock_Padding3,
> BufFreelistLock_Padding4,
> BufFreelistLock_Padding5,
> BufFreelistLock_Padding6,
> BufFreelistLock_Padding7,
> LockMgrLock,
> LockMgrLock_Padding1,
> LockMgrLock_Padding2,
> LockMgrLock_Padding3,
> LockMgrLock_Padding4,
> LockMgrLock_Padding5,
> LockMgrLock_Padding6,
> LockMgrLock_Padding7,
> OidGenLock,
> XidGenLock,
> ProcArrayLock,
> SInvalLock,
> FreeSpaceLock,
> WALInsertLock,
> WALInsertLock_Padding1,
> WALInsertLock_Padding2,
> WALInsertLock_Padding3,
> WALInsertLock_Padding4,
> WALInsertLock_Padding5,
> WALInsertLock_Padding6,
> WALInsertLock_Padding7,
> WALWriteLock,
> WALWriteLock_Padding1,
> WALWriteLock_Padding2,
> WALWriteLock_Padding3,
> WALWriteLock_Padding4,
> WALWriteLock_Padding5,
> WALWriteLock_Padding6,
> WALWriteLock_Padding7,
> ...
>
> where the number of padding locks is determined by how many lock
> structures fit within a 128 byte cache line.
>
> This isn't exactly elegant coding, but it provides a useful improvement
> on an 8-way SMP box when run on 8.0 base. OK, lets be brutal: this looks
> pretty darn stupid. But it does follow the CPU optimization handbook
> advice and I did see a noticeable improvement in performance and a
> reduction in context switching.
>
> I'm not in a position to try this again now on 8.1beta, but I'd welcome
> a performance test result from anybody that is. I'll supply a patch
> against 8.1beta for anyone wanting to test this.
>
> Best Regards, Simon Riggs
>
>
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