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Re: [Qemu-devel] [PATCH] qcow2: allocate cluster_cache/cluster_data on d


From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] [PATCH] qcow2: allocate cluster_cache/cluster_data on demand
Date: Sat, 19 Aug 2017 12:53:46 +1000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 19/08/17 12:46, Alexey Kardashevskiy wrote:
> On 19/08/17 01:18, Eric Blake wrote:
>> On 08/18/2017 08:31 AM, Stefan Hajnoczi wrote:
>>> Most qcow2 files are uncompressed so it is wasteful to allocate (32 + 1)
>>> * cluster_size + 512 bytes upfront.  Allocate s->cluster_cache and
>>> s->cluster_data when the first read operation is performance on a
>>> compressed cluster.
>>>
>>> The buffers are freed in .bdrv_close().  .bdrv_open() no longer has any
>>> code paths that can allocate these buffers, so remove the free functions
>>> in the error code path.
>>>
>>> Reported-by: Alexey Kardashevskiy <address@hidden>
>>> Cc: Kevin Wolf <address@hidden>
>>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>>> ---
>>> Alexey: Does this improve your memory profiling results?
>>
>> Is this a regression from earlier versions? 
> 
> Hm, I have not thought about this.
> 
> So. I did bisect and this started happening from
> 9a4c0e220d8a4f82b5665d0ee95ef94d8e1509d5
> "hw/virtio-pci: fix virtio behaviour"
> 
> Before that, the very same command line would take less than 1GB of
> resident memory. That thing basically enforces virtio-1.0 for QEMU <=2.6
> which means that upstream with "-machine pseries-2.6" works fine (less than
> 1GB), "-machine pseries-2.7" does not (close to 7GB, sometime even 9GB).
> 
> Then I tried bisecting again, with
> "scsi=off,disable-modern=off,disable-legacy=on" on my 150 virtio-block
> devices, started from
> e266d421490e0 "virtio-pci: add flags to enable/disable legacy/modern" (it
> added the disable-modern switch) which uses 2GB of memory.
> 
> I ended up with ada434cd0b44 "virtio-pci: implement cfg capability".
> 
> Then I removed proxy->modern_as on v2.10.0-rc3 (see below) and got 1.5GB of
> used memory (yay!)
> 
> I do not really know how to reinterpret all of this, do you?
> 
> 
> Note: 1GB..9GB numbers from below are the peak values from valgrind's

s/from below/from above/ , sorry, bad cut-n-paste :)


-- 
Alexey

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