Re: Speed up Clog Access by increasing CLOG buffers

On Fri, Sep 11, 2015 at 9:21 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Fri, Sep 11, 2015 at 10:31 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > Could you perhaps try to create a testcase where xids are accessed that
> > > are so far apart on average that they're unlikely to be in memory? And
> > > then test that across a number of client counts?
> > >
> >
> > Now about the test, create a table with large number of rows (say 11617457,
> > I have tried to create larger, but it was taking too much time (more than a day))
> > and have each row with different transaction id.  Now each transaction should
> > update rows that are at least 1048576 (number of transactions whose status can
> > be held in 32 CLog buffers) distance apart, that way ideally for each update it will
> > try to access Clog page that is not in-memory, however as the value to update
> > is getting selected randomly and that leads to every 100th access as disk access.
>
> What about just running a regular pgbench test, but hacking the
> XID-assignment code so that we increment the XID counter by 100 each
> time instead of 1?
>

If I am not wrong we need 1048576 number of transactions difference

for each record to make each CLOG access a disk access, so if we

increment XID counter by 100, then probably every 10000th (or multiplier

of 10000) transaction would go for disk access.

The number 1048576 is derived by below calc:

#define CLOG_XACTS_PER_BYTE 4
#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)

Transaction difference required for each transaction to go for disk access:

CLOG_XACTS_PER_PAGE * num_clog_buffers.

I think reducing to every 100th access for transaction status as disk access

is sufficient to prove that there is no regression with the patch for the screnario

asked by Andres or do you think it is not?

Now another possibility here could be that we try by commenting out fsync

in CLOG path to see how much it impact the performance of this test and

then for pgbench test.  I am not sure there will be any impact because even

every 100th transaction goes to disk access that is still less as compare

WAL fsync which we have to perform for each transaction.