Re: MultiXact\SLRU buffers configuration

public inbox for [email protected]  
help / color / mirror / Atom feed

From: Andrey Borodin <[email protected]>
To: Thomas Munro <[email protected]>
Cc: Gilles Darold <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: Alexander Korotkov <[email protected]>
Cc: Anastasia Lubennikova <[email protected]>
Cc: Daniel Gustafsson <[email protected]>
Cc: Kyotaro Horiguchi <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: MultiXact\SLRU buffers configuration
Date: Sat, 27 Mar 2021 10:31:54 +0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CA+hUKGKVqrxOp82zER1=XN=yPwV_-OCGAg=ez=1iz9rG+A7Smw@mail.gmail.com>
References: <[email protected]>
	<20201028013651.de5cj2xadgmba5nf@development>
	<[email protected]>
	<20201028233243.ygm6yqlynkqpzekr@development>
	<[email protected]>
	<20201029134933.xd4mh2cofuf6tdfz@development>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<CA+hUKGJ4TwipnMDmKdORu4zUo_rofsuu5FQsgV=UHJ3bY38ERw@mail.gmail.com>
	<CA+hUKGK2bkx91=CWRuPqWdXAu2JSS0mDPBh8DNuSeKhy8eOLgg@mail.gmail.com>
	<CA+hUKGL0g+Nmf=2XePaXtw82BkW9CRzoxxXB4gqZOk7tHxNJXg@mail.gmail.com>
	<[email protected]>
	<[email protected]>
	<CA+hUKGKVqrxOp82zER1=XN=yPwV_-OCGAg=ez=1iz9rG+A7Smw@mail.gmail.com>



> 27 марта 2021 г., в 01:26, Thomas Munro <[email protected]> написал(а):
> 
> On Sat, Mar 27, 2021 at 4:52 AM Andrey Borodin <[email protected]> wrote:
>> Some thoughts on HashTable patch:
>> 1. Can we allocate bigger hashtable to reduce probability of collisions?
> 
> Yeah, good idea, might require some study.
In a long run we always have this table filled with nslots. But the keys will be usually consecutive numbers (current working set of CLOG\Multis\etc). So in a happy hashing scenario collisions will only appear for some random backward jumps. I think just size = nslots * 2 will produce results which cannot be improved significantly.
And this reflects original growth strategy SH_GROW(tb, tb->size * 2).

>> 2. Can we use specialised hashtable for this case? I'm afraid hash_search() does comparable number of CPU cycles as simple cycle from 0 to 128. We could inline everything and avoid hashp->hash(keyPtr, hashp->keysize) call. I'm not insisting on special hash though, just an idea.
> 
> I tried really hard to not fall into this rabbit h.... [hack hack
> hack], OK, here's a first attempt to use simplehash,

> Andres's
> steampunk macro-based robinhood template
Sounds magnificent.

> that we're already using for
> several other things
I could not find much tests to be sure that we do not break something...

> , and murmurhash which is inlineable and
> branch-free.
I think pageno is a hash already. Why hash any further? And pages accessed together will have smaller access time due to colocation.

>  I had to tweak it to support "in-place" creation and
> fixed size (in other words, no allocators, for use in shared memory).
We really need to have a test to know what happens when this structure goes out of memory, as you mentioned below. What would be apropriate place for simplehash tests?

> Then I was annoyed that I had to add a "status" member to our struct,
> so I tried to fix that.
Indeed, sizeof(SlruMappingTableEntry) == 9 seems strange. Will simplehash align it well?


Thanks!

Best regards, Andrey Borodin.

view thread (81+ messages)  latest in thread

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]
  Subject: Re: MultiXact\SLRU buffers configuration
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox