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Date:   Sat, 29 Jun 2019 17:39:24 +0100
From:   Luke Kenneth Casson Leighton <lkcl@...l.net>
To:     torvalds@...ux-foundation.org
Cc:     akpm@...ux-foundation.org, axboe@...nel.dk,
        darrick.wong@...cle.com, david@...morbit.com, dchinner@...hat.com,
        josef@...icpanda.com, kent.overstreet@...il.com,
        linux-bcache@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org, tj@...nel.org,
        viro@...iv.linux.org.uk, zach.brown@...com
Subject: Re: bcachefs status update (it's done cooking; let's get this sucker merged)

hey linus, you made news again, all blown up and pointless again.
you're doing great: you're being honest. remember the offer i made to
put you in touch with my friend.

anecdotal story: andrew tridgell worked on the fujitsu sparc
supercomputer a couple decades ago: it had a really weird DMA ring
bus.

* memory-to-memory copy (in the same core) was 10mbytes/sec
* DMA memory-to-memory copy (in the same core) was 20mbytes/sec
* memory-memory copy (across the ring bus i.e. to another machine) was
100mbytes/sec
* DMA memory-memory copy (across the ring bus) was *200* mbytes/sec.

when andrew tried asking people, "hey everyone, we need a filesystem
that can work really well on this fast parallel system", he had to
continuously fend off "i got a great idea for in-core memory-to-memory
cacheing!!!!" suggestions, because they *just would never work*.

the point being: caches aren't always "fast".

/salutes.

l.

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