Thanks for your reply. 

Actually, the result of without "rdtsc" is reasonable. I used perf to analyze 
the performance and found that
even thought the function tracking conflicts (RWConflictExists) was faster, the 
function inserting conflicts (SetRWConflict)
was too slower. According to your suggestion, I found  there were much more 
waiting events of predicate_lock_manager.
That means, slower SetRWConflict resulted in more lock conflicts. 
So I took some effort to made it faster in the last days.  

Why the code with "rdtsc" is much faster? I thought that may be caused by some 
When I changed a machine to run the code, this phenomenon didn't happen 
-----Original Messages-----
From: "Robert Haas" <>
Sent Time: 2017-07-29 02:46:47 (Saturday)
To: "Mengxing Liu" <>
Cc: "Alvaro Herrera" <>, kgrittn <>, 
"" <>
Subject: Re: [HACKERS] [GSOC] Eliminate O(N^2) scaling from rw-conflict 
tracking in serializable transactions

On Wed, Jul 26, 2017 at 11:41 AM, Mengxing Liu <> 

Hi, all. There was a very strange phenomenon I couldn't explain. So I was 
wondering if you can help me.

I was trying to replace the linked list with a skip list in serializable 
transaction object for faster conflict tracking. But the performance is bad.
So I used the instruction "rdtsc" to compare the speed of my skip list and the 
original linked list. The skip list was about 1.5x faster.

The interesting thing is that if I added the instruction "rdstc" at the end of 
the function "RWConflictExists", 
the performance of the whole system was increased by at most 3 times! 
Here is the result. 

| benchmarks | without rdtsc  | with rdtsc |
| simpe read/write | 4.91 | 14.16 |
| ssibench | 9.72 | 10.24 |
| tpcb | 26.45 | 26.38 |

( The simple read/write benchmark has the most number of conflicts. )

The patch is attached. All the difference of the two columns is with/without a 
simple line of code:
__asm__ __volatile__ ("rdtsc"); 
But I don't know why this instruction will influence the performance so much!

Lock contention is really expensive, so a slight delay that is just long enough 
to prevent the contention from happening can sometimes improve performance.  
This example is surprisingly dramatic, though.  Of course, we can't commit it 
this way -- it will break on non-x86.

I would suggest that you gather information on what wait events are occurring 
in the "without rdtsc" case.  Like this:

$ script

$ psql

psql=> select wait_event from pg_stat_activity;

psql=> \watch 0.5 test in another window...




...use awk or perl or something to count up the wait events and see where the 
contention is happening...


Robert Haas
The Enterprise PostgreSQL Company



Mengxing Liu

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