Hi, Alvaro and Kevin. > Anyway, this is just my analysis. > So I want to hack the PG and count the conflict lists' size of transactions. > That would be more accurate.
In the last week, I hacked the PG to add an additional thread to count RWConflict list lengths. And tune the benchmark to get more conflict. But the result is still not good. > > > > > Yeah, you need a workload that generates a longer conflict list -- if > > you can make the tool generate a conflict list with a configurable > > length, that's even better (say, 100 conflicts vs. 1000 conflicts). > > Then we can see how the conflict list processing scales. > > > > Yes, I tried to increase the read set to make more conflicts. > However the abort ratio will also increase. The CPU cycles consumed by > conflict tracking are still less than 1%. > According to the design of PG, a transaction will be aborted if there is a > rw-antidependency. > In this case, a transaction with a longer conflict list, is more possible to > be aborted. > That means, the conflict list doesn't have too many chances to grow too long. > To solve this problem, I use just two kinds of transactions: Read-only transactions and Update-only transactions. In this case, no transaction would have an In-RWconflict and an Out-RWconflict at the same time. Thus transactions would not be aborted by conflict checking. Specifically, The benchmark is like this: The table has 10K rows. Read-only transactions read 1K rows and Update-only transactions update 20 random rows of the table. In this benchmark, about 91% lists are shorter than 10; lengths of 6% conflict lists are between 10 and 20. Only 2% lists are longer than 20. The CPU utilization of CheckForSerializableConflictOut/In is 0.71%/0.69%. I tried to increase the write set. As a result, conflict list become longer. But the total CPU utilization is decreased (about 50%). CPU is not the bottleneck anymore. I'm not familiar with other part of PG. Is it caused by LOCK? Is there any chance to get rid of this problem? BTW, I found that the email is not very convenient, especially when I have some problem and want to discuss with you. Would you mind scheduling a meeting every week by Skye, or other instant message software you like? I would not take you too much time. Maybe one hour is enough. Sincerely. -- Mengxing Liu -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
