maria-discuss team mailing list archive

[JCA <1.41421@xxxxxxxxx>]

Thanks for your feedback. Please see my comments  interspersed below.

On Mon, Oct 7, 2019 at 10:38 AM Guillaume Lefranc <guillaume@xxxxxxxxxxxx>
wrote:

> Hello,
>
> thread-handling=pool-of-threads
>> max_connections = 1000
>> table_open_cache = 800
>> query_cache_type = 0
>> innodb_buffer_pool_size = 512M
>> innodb_buffer_pool_instances = 10
>> innodb_adaptive_hash_index_partitions = 20
>> innodb_lock_wait_timeout = 5000
>>
> No need to use buffer pool instances with only 512M of buffer. you said
> you have 24GB of RAM - why not increase the buffer size? how big is your
> table on the disk right now? If you want the best performance it must be
> hold in the buffer.
>

It does not seem to be that big - about 101 MB. I used the following
command:

 SELECT table_name AS `Table`, round(((data_length + index_length) / 1024 /
1024), 2) `Size (MB)` FROM information_schema.TABLES WHERE table_schema =
"<my-db-name>";

I had to look it up.


>> With this, my application can keep up with the FIFO writer, but -
>> depending on the circumstances - my database can't. As I am writing this,
>> there are over 1300 threads connected to my database; any command that I
>> issue at the mysql CLI takes over one minute to return. I am keeping track
>> on how long each thread takes to complete, and that is of the order of
>> hundreds of seconds - sometimes thousands. Each thread is itself simple, in
>> that it just issues a couple of simple MariaDB commands.  Currently my
>> table consists of 1.6 million entries, and growing - on this basis, I
>> expect that things will get only worse. Each entry,however, will never
>> require more than a couple of hundred bytes of storage. The operations that
>> can be undertaken on entries are insertion, deletion and modification, the
>> latter being straightforward - like e.g. incrementing a counter or
>> replacing a short string.
>>
>
> You are not considering many factors which is
> a) limiting the number of concurrent threads - with 1300 threads you are
> creating concurrency races and locking issues. Try limiting your threads to
> a factor of the # of cores
>

   I will. Notice, though, that most of the time most of the cores are idle
anyway.


> b), you're writing to a disk system. The number of CPU cores won't matter
> if you saturate the disk. You say nothing about the disk, if it's SSD, HDD
> etc. Note that HDD's are severely handicapped when it comes to concurrent
> IO operations.
>

   It is an HDD. I am sure it will be a factor in making things slower.


> c) given the factor above you could maybe try relaxing commit to disk if
> integrity of the data is not of utmost importance, for example by adding
> "innodb_flush_log_at_trx_commit = 2" to your config.
>
>      Thanks - I'll try that.


> My system has 24 GB of  RAM and 12 cores. Occasionally all the cores are
> fully busy with MariaDB activity, but most of the time barely one or two
> are.
>
> d) CPU in a DB system will mostly be used for queries (index scans) and
> some for reindexing but you're only doing writes, so unless you have dozens
> of indexes
> e) Table design - is your table design efficient ? how many rows per sec
> are you writing? maybe you could also benefit from hash partitioning or
> clever data types.
>
>>
>> I am a newbie when it comes to interacting with MariaDB - please,  bear
>> with me. I know I must use a single database and a single table. I also
>> know - because of the nature of the data that are being written to the FIFO
>> - that the  probability for two different threads to be operating on the
>> same entry in the table at the same time is negligible - i.e. for all
>> practical purposes, that will not happen.
>> What I need is advice on how to configure my instance of MariaDB to
>> perform optimally in the scenario above. In particular, I would like for it
>> to make better use of all the cores available - in essence, to parallelize
>> the database operations as much as possible.
>>
>> f) Congratulations, you have managed to contradict yourself in two
> sentences.
>
If your workload isn't parallel by design you will not make use of the
> available cores. Also, your workload sounds IO-bound to me - there's a
> strong change the disk is the bottleneck.
> g) "I know I must use a single database and a single table." How do you
> know this if you are a newbie? No offense meant but nost of monolithic
> design is not the best to leverage performance....
>
>      The data that I have consist of a string S, two string attributes T
and A, and an integer attribute D.  String S is what I have to use as a key
in order to insert/delete/modify entries. Can this be advantageously be
stored across several tables?  The S strings have nothing in common, beyond
the attributes that I mentioned. As for the parallelism, the essence is
what I pointed out - namely, that no two threads will work on the same
entry (as accessed by S) at the same time.  They can of course be adding
new entries, or deleting existing ones, at the same time, with the proviso
that the insertion operations will always be for different values of S, and
analogously for the deletion operations. I am indeed open to suggestions
about a better design.


> -GL
>