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From: Junio C Hamano <>
To: Neeraj Singh <>
Cc: Jeff Hostetler <>,, "Neeraj K. Singh" <>
Subject: Re: [PATCH] read-cache: make the index write buffer size 128K
Date: Wed, 24 Feb 2021 21:41:24 -0800	[thread overview]
Message-ID: <xmqqmtvswvp7.fsf@gitster.g> (raw)

Neeraj Singh <> writes:

> If we think about doing the fastest possible memcpy, I think we want to aim for
> maximizing the use of the CPU cache.  A write buffer that's too big would result
> in most of the data being flushed to DRAM between when git writes it and the
> OS reads it.  L1 caches are typically ~32K and L2 caches are on the
> order of 256K.
> We probably don't want to exceed the size of the L2 cache, and we
> should actually
> leave some room for OS code and data, so 128K is a good number from
> that perspective.
> I collected data from an experiment with different buffer sizes on Windows on my
> 3.6Ghz Xeon W-2133 machine:
> The timing is pretty much in the noise after we pass 32K.  So I think
> 8K is too small, but
> given the flatness of the curve we can feel good about any value above
> 32K from a performance
> perspective.  I still think 128K is a decent number that won't likely
> need to be changed for
> some time.

Thanks for a supporting graph.

I can very well imagine that it would have been tempting to instead
say "after we pass 128k" while explaining exactly the same graph,
and doing so would have given a more coherent argument to support
the choice of 128k the patch made.  You knew that a "then perhaps we
can reclaim 96k by sizing the buffer down a bit?" would become a
reasonable response, but you still chose to be honest, which I kinda
like ;-)

  reply	other threads:[~2021-02-25  5:42 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-18  2:48 [PATCH] read-cache: make the index write buffer size 128K Neeraj K. Singh via GitGitGadget
2021-02-19 19:12 ` Jeff Hostetler
2021-02-20  3:28   ` Junio C Hamano
2021-02-20  7:56     ` Neeraj Singh
2021-02-21 12:51       ` Junio C Hamano
2021-02-24 20:56         ` Neeraj Singh
2021-02-25  5:41           ` Junio C Hamano [this message]
2021-02-25  6:58             ` Chris Torek
2021-02-25  7:16               ` Junio C Hamano
2021-02-25  7:36                 ` Neeraj Singh
2021-02-25  7:57                   ` Chris Torek

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