Re: [RFC] Tracer Ring Buffer splice() vs page cache [was: Re: Perf and ftrace [was Re: PyTimechart]]

Mathieu Desnoyers
2010-05-18T15:43:30Z

> On Tue, 2010-05-18 at 11:16 -0400, Mathieu Desnoyers wrote:
> > > Also, suppose it was still in the page-cache and still dirty, a steal()
> > > would then punch a hole in the file.
> >
> > page_cache_pipe_buf_steal starts by doing a wait_on_page_writeback(page); and
> > then does a try_to_release_page(page, GFP_KERNEL). Only if that succeeds is the
> > action of stealing succeeding.
>
> If you're going to wait for writeback I don't really see the advantage
> of stealing over simply allocating a new page.

That would allow the ring buffer to use a bounded amount of memory and not
pollute the page cache uselessly. When allocating pages as you propose, the
tracer will quickly fill and pollute the page cache with trace file pages, which
will have a large impact on I/O behavior. But in 99.9999% of use-cases, we don't
ever need to access them after they have been saved to disk.

By re-stealing its own pages after waiting for the writeback to complete, the
ring buffer would use a bounded amount of pages. If larger buffers are needed,
the user just has to specify a larger buffer size.

Thanks,