* Rob van der Heij (rvdheij@xxxxxxxxx) 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.
> Your effort to help Linux mm seems a bit overkill,
Without performing this, I have a situation similar as yours, where
LTTng fills up the page cache very quickly, until it gets to a point
where memory pressure level increase enough that the consumerd is
blocked until some pages are reclaimed. I really don't care about making
the consumerd "as fast as possible for a while" if it means its
throughput will drop when the page cache is filled. I prefer a constant
slower pace to a short burst followed by slower throughput.
> and you don't want every application to do it like that himself.
Indeed, tracing has always been slightly odd in the sense that it's not
the workload the system is meant to run, but rather a tool that should
have the smallest impact on the usual system's run when it is used.
> The
> fadvise will not even work when the page is still to be flushed out.
> Without the patch that started the thread, it would 'at random' not work
> due to SMP race condition (not multi-threading).
This is why the lttng consumerd calls:
sync_file_range with flags:
SYNC_FILE_RANGE_WAIT_BEFORE
SYNC_FILE_RANGE_WRITE
SYNC_FILE_RANGE_WAIT_AFTER
on the page range. The purpose of this call is to flush the pages to
disk before calling fadvise(POSIX_FADV_DONTNEED) on the page range.
Hopefully we can manage to make the SMP race scenario work with the
semantic you want without requiring cases where the pages are on the
local CPU, and already flushed to disk, to schedule work (and wait for
it) on each CPU. We can expect this to perform very badly on 64+ core
systems.
As far as LTTng is concerned, if every call to
fadvise(POSIX_FADV_DONTNEED) involves synchronization across all CPUs, I
will reconsider using this hint, because the performance hit would start
to be worse than the benefit of the hint given to the kernel.
Your idea of batching was interesting though. Perharps it would be good
to let the kernel batch messaging to remote CPUs within fadvise(), and
perform those asynchronously. Therefore, we could return to the
application quickly without awaiting for page invalidation to be
completed. After all, this is just a hint, and AFAIU there is no need to
guarantee that all those pages are invalidated before fadvise() returns
to user-space.
Thoughts ?
Thanks,
Mathieu
>
> I believe the right way would be for Linux to implement the fadvise flags
> properly to guide cache replacement.
>
> My situation was slightly different. I run in a virtualized environment
> where it would be a global improvement for the Linux guest not to cache
> data, even though from Linux mm perspective there is plenty of space and no
> good reason not to keep the data. My scenario intercepted the close() call
> to drop each input file during a tar or file system scan. This avoids
> building a huge page cache with stuff that will not be referenced until
> next backup and thus gets paged out by the hypervisor. There's some more
> here:
> http://zvmperf.wordpress.com/2013/01/27/taming-linux-page-cache/
>
> Rob
>
>
> On 19 June 2013 21:25, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>wrote:
>
> > * Mel Gorman (mgorman@xxxxxxx) wrote:
> > > On Tue, Jun 18, 2013 at 10:29:26AM +0100, Mel Gorman wrote:
> > > > On Mon, Jun 17, 2013 at 02:24:59PM -0700, Andrew Morton wrote:
> > > > > On Mon, 17 Jun 2013 10:13:57 -0400 Mathieu Desnoyers <
> > mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > > > >
> > > > > > Hi,
> > > > > >
> > > > > > CCing lkml on this,
> > > > > >
> > > > > > * Yannick Brosseau (yannick.brosseau@xxxxxxxxx) wrote:
> > > > > > > Hi all,
> > > > > > >
> > > > > > > We discovered a performance regression in recent kernels with
> > LTTng
> > > > > > > related to the use of fadvise DONTNEED.
> > > > > > > A call to this syscall is present in the LTTng consumer.
> > > > > > >
> > > > > > > The following kernel commit cause the call to fadvise to be
> > sometime
> > > > > > > really slower.
> > > > > > >
> > > > > > > Kernel commit info:
> > > > > > > mm/fadvise.c: drain all pagevecs if POSIX_FADV_DONTNEED fails to
> > discard
> > > > > > > all pages
> > > > > > > main tree: (since 3.9-rc1)
> > > > > > > commit 67d46b296a1ba1477c0df8ff3bc5e0167a0b0732
> > > > > > > stable tree: (since 3.8.1)
> > > > > > >
> > https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit?id=bb01afe62feca1e7cdca60696f8b074416b0910d
> > > > > > >
> > > > > > > On the workload test, we observe that the call to fadvise takes
> > about
> > > > > > > 4-5 us before this patch is applied. After applying the patch,
> > The
> > > > > > > syscall now takes values from 5 us up to 4 ms (4000 us)
> > sometime. The
> > > > > > > effect on lttng is that the consumer is frozen for this long
> > period
> > > > > > > which leads to dropped event in the trace.
> > > > >
> > > > > That change wasn't terribly efficient - if there are any unpopulated
> > > > > pages in the range (which is quite likely), fadvise() will now always
> > > > > call invalidate_mapping_pages() a second time.
> > > > >
> > > >
> > > > I did not view the path as being performance critical and did not
> > anticipate
> > > > a delay that severe.
> > >
> > > Which I should have, schedule_on_each_cpu is documented to be slow.
> > >
> > > > The original test case as well was based on
> > > > sequential IO as part of a backup so I was also generally expecting the
> > > > range to be populated. I looked at the other users of
> > lru_add_drain_all()
> > > > but there are fairly few. compaction uses them but only when used via
> > sysfs
> > > > or proc. ksm uses it but it's not likely to be that noticable. mlock
> > uses
> > > > it but it's unlikely it is being called frequently so I'm not going to
> > > > worry performance of lru_add_drain_all() in general. I'll look closer
> > at
> > > > properly detecting when it's necessarily to call in the fadvise case.
> > > >
> > >
> > > This compile-only tested prototype should detect remaining pages in the
> > rage
> > > that were not invalidated. This will at least detect unpopulated pages
> > but
> > > whether it has any impact depends on what lttng is invalidating. If it's
> > > invalidating a range of per-cpu traces then I doubt this will work
> > because
> > > there will always be remaining pages. Similarly I wonder if passing down
> > > the mapping will really help if a large number of cpus are tracing as we
> > > end up scheduling work on every CPU regardless.
> >
> > Here is how LTTng consumerd is using POSIX_FADV_DONTNEED. Please let me
> > know if we are doing something stupid. ;-)
> >
> > The LTTng consumerd deals with trace packets, which consists of a set of
> > pages. I'll just discuss how the pages are moved around, leaving out
> > discussion about synchronization between producer/consumer.
> >
> > Whenever the Nth trace packet is ready to be written to disk:
> >
> > 1) splice the entire set of pages of trace packet N to disk through a
> > pipe,
> > 2) sync_file_range on trace packet N-1 with the following flags:
> > SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE |
> > SYNC_FILE_RANGE_WAIT_AFTER
> > so we wait (blocking) on the _previous_ trace packet to be completely
> > written to disk.
> > 3) posix_fadvise POSIX_FADV_DONTNEED on trace packet N-1.
> >
> > There are a couple of comments in my consumerd code explaining why we do
> > this sequence of steps:
> >
> > /*
> > * Give hints to the kernel about how we access the file:
> > * POSIX_FADV_DONTNEED : we won't re-access data in a near future
> > after
> > * we write it.
> > *
> > * We need to call fadvise again after the file grows because the
> > * kernel does not seem to apply fadvise to non-existing parts of
> > the
> > * file.
> > *
> > * Call fadvise _after_ having waited for the page writeback to
> > * complete because the dirty page writeback semantic is not well
> > * defined. So it can be expected to lead to lower throughput in
> > * streaming.
> > */
> >
> > Currently, the lttng-consumerd is single-threaded, but we plan to
> > re-introduce multi-threading, and per-cpu affinity, in a near future.
> >
> > Yannick will try your patch tomorrow and report whether it improves
> > performance or not,
> >
> > Thanks!
> >
> > Mathieu
> >
> > >
> > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > index 2c28271..e2bb47e 100644
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -2176,6 +2176,8 @@ extern int invalidate_partition(struct gendisk *,
> > int);
> > > #endif
> > > unsigned long invalidate_mapping_pages(struct address_space *mapping,
> > > pgoff_t start, pgoff_t end);
> > > +unsigned long invalidate_mapping_pages_check(struct address_space
> > *mapping,
> > > + pgoff_t start, pgoff_t end);
> > >
> > > static inline void invalidate_remote_inode(struct inode *inode)
> > > {
> > > diff --git a/mm/fadvise.c b/mm/fadvise.c
> > > index 7e09268..0579e60 100644
> > > --- a/mm/fadvise.c
> > > +++ b/mm/fadvise.c
> > > @@ -122,7 +122,9 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset,
> > loff_t len, int advice)
> > > end_index = (endbyte >> PAGE_CACHE_SHIFT);
> > >
> > > if (end_index >= start_index) {
> > > - unsigned long count =
> > invalidate_mapping_pages(mapping,
> > > + unsigned long nr_remaining;
> > > +
> > > + nr_remaining =
> > invalidate_mapping_pages_check(mapping,
> > > start_index, end_index);
> > >
> > > /*
> > > @@ -131,7 +133,7 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset,
> > loff_t len, int advice)
> > > * a per-cpu pagevec for a remote CPU. Drain all
> > > * pagevecs and try again.
> > > */
> > > - if (count < (end_index - start_index + 1)) {
> > > + if (nr_remaining) {
> > > lru_add_drain_all();
> > > invalidate_mapping_pages(mapping,
> > start_index,
> > > end_index);
> > > diff --git a/mm/truncate.c b/mm/truncate.c
> > > index c75b736..86cfc2e 100644
> > > --- a/mm/truncate.c
> > > +++ b/mm/truncate.c
> > > @@ -312,26 +312,16 @@ void truncate_inode_pages(struct address_space
> > *mapping, loff_t lstart)
> > > }
> > > EXPORT_SYMBOL(truncate_inode_pages);
> > >
> > > -/**
> > > - * invalidate_mapping_pages - Invalidate all the unlocked pages of one
> > inode
> > > - * @mapping: the address_space which holds the pages to invalidate
> > > - * @start: the offset 'from' which to invalidate
> > > - * @end: the offset 'to' which to invalidate (inclusive)
> > > - *
> > > - * This function only removes the unlocked pages, if you want to
> > > - * remove all the pages of one inode, you must call
> > truncate_inode_pages.
> > > - *
> > > - * invalidate_mapping_pages() will not block on IO activity. It will not
> > > - * invalidate pages which are dirty, locked, under writeback or mapped
> > into
> > > - * pagetables.
> > > - */
> > > -unsigned long invalidate_mapping_pages(struct address_space *mapping,
> > > - pgoff_t start, pgoff_t end)
> > > +static void __invalidate_mapping_pages(struct address_space *mapping,
> > > + pgoff_t start, pgoff_t end,
> > > + unsigned long *ret_nr_invalidated,
> > > + unsigned long *ret_nr_remaining)
> > > {
> > > struct pagevec pvec;
> > > pgoff_t index = start;
> > > unsigned long ret;
> > > - unsigned long count = 0;
> > > + unsigned long nr_invalidated = 0;
> > > + unsigned long nr_remaining = 0;
> > > int i;
> > >
> > > /*
> > > @@ -354,8 +344,10 @@ unsigned long invalidate_mapping_pages(struct
> > address_space *mapping,
> > > if (index > end)
> > > break;
> > >
> > > - if (!trylock_page(page))
> > > + if (!trylock_page(page)) {
> > > + nr_remaining++;
> > > continue;
> > > + }
> > > WARN_ON(page->index != index);
> > > ret = invalidate_inode_page(page);
> > > unlock_page(page);
> > > @@ -365,17 +357,73 @@ unsigned long invalidate_mapping_pages(struct
> > address_space *mapping,
> > > */
> > > if (!ret)
> > > deactivate_page(page);
> > > - count += ret;
> > > + else
> > > + nr_remaining++;
> > > + nr_invalidated += ret;
> > > }
> > > pagevec_release(&pvec);
> > > mem_cgroup_uncharge_end();
> > > cond_resched();
> > > index++;
> > > }
> > > - return count;
> > > +
> > > + *ret_nr_invalidated = nr_invalidated;
> > > + *ret_nr_remaining = nr_remaining;
> > > }
> > > EXPORT_SYMBOL(invalidate_mapping_pages);
> > >
> > > +/**
> > > + * invalidate_mapping_pages - Invalidate all the unlocked pages of one
> > inode
> > > + * @mapping: the address_space which holds the pages to invalidate
> > > + * @start: the offset 'from' which to invalidate
> > > + * @end: the offset 'to' which to invalidate (inclusive)
> > > + *
> > > + * This function only removes the unlocked pages, if you want to
> > > + * remove all the pages of one inode, you must call
> > truncate_inode_pages.
> > > + *
> > > + * invalidate_mapping_pages() will not block on IO activity. It will not
> > > + * invalidate pages which are dirty, locked, under writeback or mapped
> > into
> > > + * pagetables.
> > > + *
> > > + * Returns the number of pages invalidated
> > > + */
> > > +unsigned long invalidate_mapping_pages(struct address_space *mapping,
> > > + pgoff_t start, pgoff_t end)
> > > +{
> > > + unsigned long nr_invalidated, nr_remaining;
> > > +
> > > + __invalidate_mapping_pages(mapping, start, end,
> > > + &nr_invalidated, &nr_remaining);
> > > +
> > > + return nr_invalidated;
> > > +}
> > > +
> > > +/**
> > > + * invalidate_mapping_pages_check - Invalidate all the unlocked pages
> > of one inode and check for remaining pages.
> > > + * @mapping: the address_space which holds the pages to invalidate
> > > + * @start: the offset 'from' which to invalidate
> > > + * @end: the offset 'to' which to invalidate (inclusive)
> > > + *
> > > + * This function only removes the unlocked pages, if you want to
> > > + * remove all the pages of one inode, you must call
> > truncate_inode_pages.
> > > + *
> > > + * invalidate_mapping_pages() will not block on IO activity. It will not
> > > + * invalidate pages which are dirty, locked, under writeback or mapped
> > into
> > > + * pagetables.
> > > + *
> > > + * Returns the number of pages remaining in the invalidated range
> > > + */
> > > +unsigned long invalidate_mapping_pages_check(struct address_space
> > *mapping,
> > > + pgoff_t start, pgoff_t end)
> > > +{
> > > + unsigned long nr_invalidated, nr_remaining;
> > > +
> > > + __invalidate_mapping_pages(mapping, start, end,
> > > + &nr_invalidated, &nr_remaining);
> > > +
> > > + return nr_remaining;
> > > +}
> > > +
> > > /*
> > > * This is like invalidate_complete_page(), except it ignores the page's
> > > * refcount. We do this because invalidate_inode_pages2() needs
> > stronger
> >
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > http://www.efficios.com
> >
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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