ChallengeSpot the optimization–solution
In my previous post, I asked why this change would result in a better performing system, since the total amount of work that is done is the same:
The answer is quite simple. The amount of work that our code is doing is the same, sure, but that isn’t all the code that runs.
In the first version, we would allocate the string, and then we’ll start a bunch of async operations. Those operations are likely to take some time and involve I/O (otherwise, they wouldn’t be async).
It is very likely that in the meantime, we’ll get a GC run. At that point, the string pointed to be the ids variable will be promoted (since it survived a GC). That means that it would be collected much later.
Using the new code, the scope of the ids string is far shorter. That means that the GC is more likely to catch it very early and significantly reduce the cost of releasing the memory.
More posts in "Challenge" series:
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- (14 Dec 2022) What does this code print?
- (01 Jul 2022) Find the stack smash bug… – answer
- (30 Jun 2022) Find the stack smash bug…
- (03 Jun 2022) Spot the data corruption
- (06 May 2022) Spot the optimization–solution
- (05 May 2022) Spot the optimization
- (06 Apr 2022) Why is this code broken?
- (16 Dec 2021) Find the slow down–answer
- (15 Dec 2021) Find the slow down
- (03 Nov 2021) The code review bug that gives me nightmares–The fix
- (02 Nov 2021) The code review bug that gives me nightmares–the issue
- (01 Nov 2021) The code review bug that gives me nightmares
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- (21 Apr 2020) Generate matching shard id–answer
- (20 Apr 2020) Generate matching shard id
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- (28 Sep 2018) The loop that leaks–Answer
- (27 Sep 2018) The loop that leaks
- (03 Apr 2018) The invisible concurrency bug–Answer
- (02 Apr 2018) The invisible concurrency bug
- (31 Jan 2018) Find the bug in the fix–answer
- (30 Jan 2018) Find the bug in the fix
- (19 Jan 2017) What does this code do?
- (26 Jul 2016) The race condition in the TCP stack, answer
- (25 Jul 2016) The race condition in the TCP stack
- (28 Apr 2015) What is the meaning of this change?
- (26 Sep 2013) Spot the bug
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- (17 Oct 2011) Minimum number of round trips
- (23 Aug 2011) Recent Comments with Future Posts
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- (29 Apr 2011) Stop the leaks
- (23 Dec 2010) This code should never hit production
- (17 Dec 2010) Your own ThreadLocal
- (03 Dec 2010) Querying relative information with RavenDB
- (29 Jun 2010) Find the bug
- (23 Jun 2010) Dynamically dynamic
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- (04 Mar 2010) Robust enumeration over external code
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- (21 Oct 2009) Can you spot the bug?
- (18 Oct 2009) Why is this wrong?
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- (11 May 2009) The regex that doesn’t match
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- (13 Mar 2009) C# Rewriting
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- (04 Sep 2008) Don't stop with the first DSL abstraction
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- (26 Jul 2008) Find the bug fix
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- (03 Jul 2008) Find the bug
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- (27 May 2008) Striving for better syntax
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- (21 Jan 2008) Strongly typing weakly typed code
- (28 Jun 2007) Windsor Null Object Dependency Facility
Question: Items are iterated 2 times: in the Select and again in the foreach. Why not concatenating the Ids in the foreach? No Select at all?
The cost of iterating twice on the list (since it in memory) is pretty low. You could combine these iterations, but they aren't really that costly.
But isn't the logging statement at the end causing the scope of the string variable to be extended?
Yes, but the scope is from the creation to the log, so very small duration after this change.
In your question the statement was 'we can improve the performance of this code significantly'. So we have an async function which is executing so long that the objects allocated at the start will go to Gen2 and we can 'significantly reduce the cost' by an obscure optimization. Seriously?
If this is called enough, absolutely. The issue isn't so much that it is executed so long, the issue is that this is called enough times that statistically at GC time you'll have many such instances running async and moved to next gen.That is something that you can easily observe, because you'll see 80% - 90% CPU time goes on GC, and the reason for that is that it cleans Gen2 very rarely.