Date: Sun, 13 Jul 2014 05:55:00 +0200 From: Mateusz Guzik <mjguzik@gmail.com> To: freebsd-arch@freebsd.org Cc: kib@freebsd.org Subject: Getting rid of atomic_load_acq_int(&fdp->fd_nfiles)) from fget_unlocked Message-ID: <20140713035500.GC16884@dft-labs.eu>
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Currently:
/*
* Avoid reads reordering and then a first access to the
* fdp->fd_ofiles table which could result in OOB operation.
*/
if (fd < 0 || fd >= atomic_load_acq_int(&fdp->fd_nfiles))
return (EBADF);
However, if we put fd_nfiles and fd_otable into one atomically replaced
structure the only need to:
1. make sure the pointer is read once
2. issue a data dependency barrier - this is a noop on all supported
architectures and we don't even have approprate macro, so doing nothing
seems fine
The motivation is to boost performance to amortize for seqlock cost, in
case it hits the tree.
This has no impact on races with capability lookup.
In a microbenchmark of 16 threads reading from the same pipe fd
immediately returning EAGAIN the numbers are:
x vanilla-readpipe-run-sum
+ noacq-readpipe-run-sum
[..]
N Min Max Median Avg Stddev
x 20 13133671 14900364 13893331 13827075 471500.82
+ 20 59479718 59527286 59496714 59499504 13752.968
Difference at 95.0% confidence
4.56724e+07 +/- 213483
330.312% +/- 1.54395%
There are 3 steps:
1. tidy up capsicum to accept fde:
http://people.freebsd.org/~mjg/patches/single-fdtable-read-capsicum.patch
2. add __READ_ONCE:
http://people.freebsd.org/~mjg/patches/read-once.patch
3. put stuff into one structure:
http://people.freebsd.org/~mjg/patches/filedescenttable.patch
Comments?
--
Mateusz Guzik <mjguzik gmail.com>
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