Re: [Qemu-devel] [PATCHv3 RESEND] block: introduce BDRV_O_SEQUENTIAL

[Peter Lieven]

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