Am 04.06.2014 17:12, schrieb Stefan Hajnoczi: > On Fri, May 30, 2014 at 11:40:37PM +0200, Peter Lieven wrote: >> this patch introduces a new flag to indicate that we are going to >> sequentially >> read from a file and do not plan to reread/reuse the data after it has been >> read. >> >> The current use of this flag is to open the source(s) of a qemu-img convert >> process. If a protocol from block/raw-posix.c is used posix_fadvise is >> utilized >> to advise to the kernel that we are going to read sequentially from the >> file and a POSIX_FADV_DONTNEED advise is issued after each write to indicate >> that there is no advantage keeping the blocks in the buffers. >> >> Consider the following test case that was created to confirm the behaviour of >> the new flag: >> >> A 10G logical volume was created and filled with random data. >> Then the logical volume was exported via qemu-img convert to an iscsi target. >> Before the export was started all caches of the linux kernel where dropped. >> >> Old behavior: >> - The convert process took 3m45s and the buffer cache grew up to 9.67 GB >> close >> to the end of the conversion. After qemu-img terminated all the buffers >> were >> freed by the kernel. >> >> New behavior with the -N switch: >> - The convert process took 3m43s and the buffer cache grew up to 15.48 MB >> close >> to the end with some small peaks up to 30 MB during the conversion. > FADVISE_SEQUENTIAL can be good since it doubles read-ahead on Linux. > > I'm skeptical of the effort to avoid buffer cache usage using > FADVISE_DONTNEED. The performance results tell me that less buffer > cache was used but that number doesn't have a direct effect on > application performance. > > Let's check GNU coreutils: > > $ cd coreutils > $ git grep FADVISE_DONTNEED > gl/lib/fadvise.h: FADVISE_DONTNEED = POSIX_FADV_DONTNEED, > gl/lib/fadvise.h: FADVISE_DONTNEED, > $ > > GNU cp(1) does not care about minimizing impact on buffer cache using > FADVISE_DONTNEED. It just sets FADVISE_SEQUENTIAL on the source file > and calls read() (plus uses FIEMAP to check extents for sparseness). > > I want to avoid adding code just for the heck of it. We need a deeper > understanding: > > Please drop FADVISE_DONTNEED and compare again to see if it changes the > benchmark. > > By the way, did you perform several runs to check the variance of the > running time? I don't know if the 2 seconds difference were noise or > because FADVISE_SEQUENTIAL or because FADVISE_DONTNEED or because both.
There was no effect on the runtime as far as I remember. I ran some tests, but not a number large enough to filter out the noise. I created this one because we saw it helps under memory pressure. Maybe its too specific to add it into mainline qemu, but I wanted to avoid to have too much individual changes we need to maintain. > >> diff --git a/block/raw-posix.c b/block/raw-posix.c >> index 6586a0c..9768cc4 100644 >> --- a/block/raw-posix.c >> +++ b/block/raw-posix.c >> @@ -447,6 +447,13 @@ static int raw_open_common(BlockDriverState *bs, QDict >> *options, >> } >> #endif >> >> +#ifdef POSIX_FADV_SEQUENTIAL >> + if (bs->open_flags & BDRV_O_SEQUENTIAL && >> + !(bs->open_flags & BDRV_O_NOCACHE)) { >> + posix_fadvise(s->fd, 0, 0, POSIX_FADV_SEQUENTIAL); >> + } >> +#endif > This is only true if the image format is raw. If the image format on > top of this raw-posix BDS is non-raw then the read pattern may not be > sequential. You are right, but will the other formats set BDRV_O_SEQUENTIAL? > > Perhaps the extra I/O in that case doesn't matter but conceptually it's > wrong to think that a raw-posix file will have a sequential access > pattern just because bdrv_read() is called sequentially. Peter