[lttng-dev] [-stable 3.8.1 performance regression] madvise POSIX_FADV_DONTNEED

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Tue Jul 2 09:58:58 EDT 2013

* Dave Chinner (david at fromorbit.com) wrote:
> On Thu, Jun 20, 2013 at 08:20:16AM -0400, Mathieu Desnoyers wrote:
> > * Rob van der Heij (rvdheij at gmail.com) wrote:
> > > Wouldn't you batch the calls to drop the pages from cache rather than drop
> > > one packet at a time?
> > 
> > By default for kernel tracing, lttng's trace packets are 1MB, so I
> > consider the call to fadvise to be already batched by applying it to 1MB
> > packets rather than indivitual pages. Even there, it seems that the
> > extra overhead added by the lru drain on each CPU is noticeable.
> > 
> > Another reason for not batching this in larger chunks is to limit the
> > impact of the tracer on the kernel page cache. LTTng limits itself to
> > its own set of buffers, and use the page cache for what is absolutely
> > needed to perform I/O, but no more.
> I think you are doing it wrong. This is a poster child case for
> using Direct IO and completely avoiding the page cache altogether....

I just tried replacing my sync_file_range()+fadvise() calls and instead
pass the O_DIRECT flag to open(). Unfortunately, I must be doing
something very wrong, because I get only 1/3rd of the throughput, and
the page cache fills up. Any idea why ?

Here are my results:

heavy-syscall.c: 30M sigaction() syscall with bad parameters (returns
immediately). Used as high-throughput stress-test for the tracer.
Tracing to disk with LTTng, all kernel tracepoints activated, including
system calls.

Tracer configuration: per-core buffers split into 4 sub-buffers of
262kB. splice() is used to transfer data from buffers to disk. Runs on a
8-core Intel machine.

Writing to a software raid-1 ext3 partition.
ext3 mount options: rw,errors=remount-ro

* sync_file_range+fadvise 3.9.8
  - with lru drain on fadvise

Kernel cache usage:
Before tracing: 56272k cached
After tracing:  56388k cached

939M	/root/lttng-traces/auto-20130702-090430
time ./heavy-syscall 
real	0m21.910s
throughput: 42MB/s

* sync_file_range+fadvise 3.9.8
  - without lru drain on fadvise: manually reverted

Kernel cache usage:
Before tracing: 67968k cached
After tracing:  67984k cached

945M	/root/lttng-traces/auto-20130702-092505
time ./heavy-syscall 
real	0m21.872s
throughput: 43MB/s

* O_DIRECT 3.9.8
  - O_DIRECT flag on open(), removed fadvise and sync_file_range calls

Kernel cache usage:
Before tracing:  99480k cached
After tracing:  360132k cached

258M	/root/lttng-traces/auto-20130702-090603
time ./heavy-syscall 
real	0m19.627s
throughput: 13MB/s

* No cache hints 3.9.8
  - only removed fadvise and sync_file_range calls

Kernel cache usage:
Before tracing: 103556k cached
After tracing:  363712k cached

945M	/root/lttng-traces/auto-20130702-092505
time ./heavy-syscall 
real	0m19.672s
throughput: 48MB/s

Thoughts ?



Mathieu Desnoyers
EfficiOS Inc.

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