Subject: misc. MMU: NUMA, big pages, idle zero, ring buffers, PAE, ...
To: None <email@example.com>
From: Edward B. DREGER <firstname.lastname@example.org>
Date: 04/29/2006 05:23:37
[ Apologies for earlier cross-posting. No idea why I sent to -perform
instead of -kern. ]
Note: Some of these ramblings are ia32/aa64-focused, but the principles
While exploring PAE last November, I wound up browsing through uvm/pmap
code. I've had a few additional ideas, and would like some [more]
/* Big Pages */
Begin by allocating memory stride 2M/4M (former iff PAE, latter iff
!PAE). Track wasted 4K [sub]pages. Split big pages into smaller ones
when needed, but avoid using page tables until then. Coalesce smaller
pages into bigger ones when free RAM permits.
Rationale: Hopefully less MMU management overhead and fewer TLB misses
while memory is plentiful. Fall back to standard behavior when needed.
/* Fractional/Checkpointed Zeroing of Big Pages */
I whipped up a crude program that performed 1000 bzero(3) iterations on
a 2M chunk. Each iteration took about 9 ms on a PIII/500 notebook.
Should the idle-zero loop zero a fraction of a big page? What about
dedicating a PDE slot (Intel terminology) to the zero code?
Rationale: Several milliseconds -- although certainly less than 9 ms
when on faster CPU and with optimized zeroing code -- is an eternity.
/* Per-CPU Management */
Both of the above, as well as free page lists, should be per-CPU. Can a
CPU be forced to work with the memory closest to it? (Consider NUMA
performance, such as multiprocessor Opteron systems.)
Rationale: Reduced inter-CPU contention. Assuming processes have
significant CPU affininty, using "nearby" memory would reduce reduce
both interconnect bandwidth use and memory access time.
/* Ring Buffers */
A native mapping for ring buffers would be nice:
u_char *ringbuf = mmapringbuf(..., MAP_RINGBUF, ...) ;
would allocate a memory region from <base> to <base + 2 * size>. i.e.,
base + size
would both be aliased to the same physical pages. Voila! Simple,
linear ringbuf where the MMU handles wraparound at the region's end.
Rationale: It's just so much easier this way. :-)
/* mremap() */
Zero-copy allocation-size changes are convenient.
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