On Mon, Oct 5, 2015 at 6:34 AM, Jeff Janes <jeff.ja...@gmail.com> wrote:

> On Fri, Sep 11, 2015 at 8:01 PM, Amit Kapila <amit.kapil...@gmail.com>
> wrote:
>> 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
>> Transaction difference required for each transaction to go for disk
>> access:
>> CLOG_XACTS_PER_PAGE * num_clog_buffers.
> That guarantees that every xid occupies its own 32-contiguous-pages chunk
> of clog.
> But clog pages are not pulled in and out in 32-page chunks, but one page
> chunks.  So you would only need 32,768 differences to get every real
> transaction to live on its own clog page, which means every look up of a
> different real transaction would have to do a page replacement.

Agreed, but that doesn't effect the test result with the test done above.

>  (I think your references to disk access here are misleading.  Isn't the
> issue here the contention on the lock that controls the page replacement,
> not the actual IO?)
The point is that if there is no I/O needed, then all the read-access for
transaction status will just use Shared locks, however if there is an I/O,
then it would need an Exclusive lock.

> I've attached a patch that allows you set the guc "JJ_xid",which makes it
> burn the given number of xids every time one new one is asked for.  (The
> patch introduces lots of other stuff as well, but I didn't feel like
> ripping the irrelevant parts out--if you don't set any of the other gucs it
> introduces from their defaults, they shouldn't cause you trouble.)  I think
> there are other tools around that do the same thing, but this is the one I
> know about.  It is easy to drive the system into wrap-around shutdown with
> this, so lowering autovacuum_vacuum_cost_delay is a good idea.
> Actually I haven't attached it, because then the commitfest app will list
> it as the patch needing review, instead I've put it here
> https://drive.google.com/file/d/0Bzqrh1SO9FcERV9EUThtT3pacmM/view?usp=sharing
Thanks, I think probably this could also be used for testing.

> 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.
> You mentioned that your clog is not on ssd, but surely at this scale of
> hardware, the hdd the clog is on has a bbu in front of it, no?

> But I thought Andres' concern was not about fsync, but about the fact that
> the SLRU does linear scans (repeatedly) of the buffers while holding the
> control lock?  At some point, scanning more and more buffers under the lock
> is going to cause more contention than scanning fewer buffers and just
> evicting a page will.

Yes, at some point, that could matter, but I could not see the impact
at 64 or 128 number of Clog buffers.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

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