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From Xiaokai Wang <xiaokai.wa...@gmail.com>
Subject Re: Locks are acquired to cost much time in transactions
Date Fri, 21 Sep 2018 05:51:55 GMT
Thanks your reply, Mike.

In our scenario, some tables have a lot of data to be updated at the same
time. The current implementation of KUDU, if TXN(transaction) can not get
all the ops lock it will wait for 1s to block the entire Tablet
processing, which
will greatly reduce the write performance of the Tablet.Reduce the size of
the batch, TXN may still have the same row key, and can not solve this
problem.


In my way, I want to change the work way of 'APPLY' phase, instead of
putting TXN to apply_pool_ queue, I will put each key-op(splitting TXN to
ops) to apply_pool_token_ queue. This will guarantee the same key's op will
be putted to the same thread. When ops belonging to the same TXN execute
over, then send rpc response and write CommitMsg to WAL. In this way, I can
abandon the keys locks.

Working follow chart just like this below:
[image: op-key_concurrent.png]
1. client send T1, T2, T3 to tserver, tserver will handle each txn in turn.

2. leader send replicated msg to follower, sending them together or
independent. T1, T2, T3 will be ordered to execute by follower and send
them back by together or independent in turn. raft_pool_token_ guarantee
T1, T2 T3 be ordered to apply.

3. Splitting Tx to ops, each op is hashed to the queue of
apply_pool_token_, key's op is order to execute as TXN turn.

--------

Kudu origin work way below:
[image: txn_concurrent.png]
1. client send T1, T2, T3 to tserver, tserver will handle each txn orderly,
they will acquire locks in 'PREPARE' phase in turn.

2. leader send replicated msg to follower, sending them together or
independent. T1, T2, T3 will be ordered to execute by follower and send
them back by together or independent in turn. raft_pool_token_ guarantee
T1, T2 T3 be ordered to apply.

3. T1, T2, T3 are putted to the queue of apply_pool_, order or concurrented
to be executed, releasing locks.

-----------

Contrast to origin, the keys locks are abandon, this can guarantee kudu
throughput more smoothly. What do you think? Hope to get your advice, kudu
users.

Thanks.
xiaokai.


From: *Mike Percy* <mpercy@apache.org>
Date: 2018年9月19日周三 上午8:29
Subject: Re: Locks are acquired to cost much time in transactions
To: <user@kudu.apache.org>


Why do you think you are spending a lot of time contending on row locks?

Have you tried configuring your clients to send smaller batches? This may
decrease throughput on a per-client basis but will likely improve latency
and reduce the likelihood of row lock contention.

If you are really spending most of your time contending on row locks then
you will likely run into more fundamental performance issues trying to
scale your writes, since Kudu's MVCC implementation effectively stores a
linked list of updates to a given cell until compaction occurs. See
https://github.com/apache/kudu/blob/master/docs/design-docs/tablet.md#historical-mvcc-in-diskrowsets
 for more information about the on-disk design.

If you accumulate too many uncompacted mutations against a given row,
reading the latest value for that row at scan time will be slow because it
has to do a lot of work at read time.

Mike

On Tue, Sep 18, 2018 at 8:48 AM Xiaokai Wang <xiaokai.wang@live.com> wrote:

Moved here from JIRA.

Hi guys, I met a problem about the keys locks that almost impacts the
service normal writing.


As we all know, a transaction which get all row_key locks will go on next
step in kudu. Everything looks good, if keys are not concurrent updated.
But when keys are updated by more than one client at the same time, locks
are acquired to wait much time. The cases are often in my product
environment. Does anybody meet the problem? Has any good ideal for this?


In my way, I want to try to abandon keys locks, instead using *_pool_token_
'SERIAL' mode which keeping the key of transaction is serial and ordered.
Dose this work?


Hope to get your advice. Thanks.


-----
Regards,
Xiaokai



-----
Regards,
Xiaokai

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