Date: Wed, 21 Mar 2001 13:38:08 -0800 (PST) From: Matt Dillon <dillon@earth.backplane.com> To: Poul-Henning Kamp <phk@critter.freebsd.dk> Cc: Mike Smith <msmith@FreeBSD.ORG>, arch@FreeBSD.ORG Subject: Re: remind me again, why is MAXPHYS only 128k ? Message-ID: <200103212138.f2LLc8a21690@earth.backplane.com> References: <89046.985209441@critter>
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When I last discussed MAXPHYS with people the issue that was predominant
was the b_pages[] array embedded in the struct buf and the pages[] arrays
declared in the VM paging code. I was worried that an increased MAXPHYS
eating too much kernel stack and too much kernel memory. We also have
the issue of the physical buffers (pbufs) reserving VM space permanently.
If you increase MAXPHYS you quickly start to hit kernel VM limitations.
There are also issues with I/O piplining and deadlock avoidance in
the VFS/BIO subsystem. The VM low memory handling is effectively
tuned to the assumption that MAXPHYS is somewhere 'around' 128K. Some
initial-value work for VM related sysctls would be required there
(minor work I think).
There are also some minor issues associated with the clustering code.
In anycase, my suggestion would be to leave it alone and instead work on
optimizing the RAW DEVICE I/O path to not be limited by MAXPHYS at all.
Increasing it to 256K might be possible, but increasing it to 1MB is
out of the question (insofar as being a default).
-Matt
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