Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

[Avi Kivity <avi@redhat.com>]

On 03/16/2010 04:27 PM, Balbir Singh wrote:
>
>> Let's assume the guest has virtio (I agree with IDE we need
>> reordering on the host).  The guest sends batches of I/O separated
>> by cache flushes.  If the batches are smaller than the virtio queue
>> length, ideally things look like:
>>
>>   io_submit(..., batch_size_1);
>>   io_getevents(..., batch_size_1);
>>   fdatasync();
>>   io_submit(..., batch_size_2);
>>    io_getevents(..., batch_size_2);
>>    fdatasync();
>>    io_submit(..., batch_size_3);
>>    io_getevents(..., batch_size_3);
>>    fdatasync();
>>
>> (certainly that won't happen today, but it could in principle).
>>
>> How does a write cache give any advantage?  The host kernel sees
>> _exactly_ the same information as it would from a bunch of threaded
>> pwritev()s followed by fdatasync().
>>
>>      
> Are you suggesting that the model with cache=writeback gives us the
> same I/O pattern as cache=none, so there are no opportunities for
> optimization?
>    

Yes.  The guest also has a large cache with the same optimization algorithm.

>
>    
>> (wish: IO_CMD_ORDERED_FDATASYNC)
>>
>> If the batch size is larger than the virtio queue size, or if there
>> are no flushes at all, then yes the huge write cache gives more
>> opportunity for reordering.  But we're already talking hundreds of
>> requests here.
>>
>> Let's say the virtio queue size was unlimited.  What
>> merging/reordering opportunity are we missing on the host?  Again we
>> have exactly the same information: either the pagecache lru + radix
>> tree that identifies all dirty pages in disk order, or the block
>> queue with pending requests that contains exactly the same
>> information.
>>
>> Something is wrong.  Maybe it's my understanding, but on the other
>> hand it may be a piece of kernel code.
>>
>>      
> I assume you are talking of dedicated disk partitions and not
> individual disk images residing on the same partition.
>    

Correct. Images in files introduce new writes which can be optimized.

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