On Fri, Feb 12, 2016 at 9:04 PM, Aleksander Alekseev < a.aleks...@postgrespro.ru> wrote:
> Hello, Robert > > > It also strikes me that it's probably quite likely that slock_t > > mutex[NUM_FREELISTS] is a poor way to lay out this data in memory. > > For example, on a system where slock_t is just one byte, most likely > > all of those mutexes are going to be in the same cache line, which > > means you're going to get a LOT of false sharing. It seems like it > > would be sensible to define a new struct that contains an slock_t, a > > long, and a HASHELEMENT *, and then make an array of those structs. > > That wouldn't completely eliminate false sharing, but it would reduce > > it quite a bit. My guess is that if you did that, you could reduce > > the number of freelists to 8 or less and get pretty much the same > > performance benefit that you're getting right now with 32. And that, > > too, seems likely to be good for single-client performance. > > I experimented for a while trying to fit every spinlock in a separate > cache line. Indeed we can gain some speedup this way. Here are > benchmark results on 12-core server for NUM_LOCK_PARTITIONS = 32 (in > this case difference is more notable): > > | FREELISTS | SIZE = 32 | SIZE = 64 | SIZE = 128 | SIZE = 256 | > |-----------|------------|------------|------------|------------| > | 4 | +25.4% | +28.7% | +28.4% | +28.3% | > | 8 | +28.2% | +29.4% | +31.7% | +31.4% | > | 16 | +29.9% | +32.6% | +31.6% | +30.8% | > | 32 | +33.7% | +34.2% | +33.6% | +32.6% | > > Here SIZE is short for FREELIST_BUFFER_SIZE (part of a hack I used to > align FREELIST structure, see attached patch). I am not sure, if this is exactly what has been suggested by Robert, so it is not straightforward to see if his suggestion can allow us to use NUM_FREELISTS as 8 rather than 32. I think instead of trying to use FREELISTBUFF, why not do it as Robert has suggested and try with different values of NUM_FREELISTS? > > > I am however wondering if it to set the freelist affinity based on > > something other than the hash value, like say the PID, so that the > > same process doesn't keep switching to a different freelist for every > > buffer eviction. > > Also I tried to use PID to determine freeList number: > > ``` > #include <sys/types.h> > #include <unistd.h> > > ... > > #define FREELIST_IDX(hctl, hashcode) \ > (IS_PARTITIONED(hctl) ? ((uint32)getpid()) % NUM_FREELISTS : 0) > > ... > > // now nentries could be negative in this case > // Assert(FREELIST(hctl, freelist_idx).nentries > 0); > > In which case, do you think entries can go negative? I think the nentries is incremented and decremented in the way as without patch, so I am not getting what can make it go negative. With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
