Re: [Qemu-devel] [PATCH v7 RFC] block/vxhs: Initial commit to add Veritas HyperScale VxHS block device support

[Ketan Nilangekar]

> On Nov 4, 2016, at 2:52 AM, Stefan Hajnoczi <address@hidden> wrote:
> 
>> On Thu, Oct 20, 2016 at 01:31:15AM +0000, Ketan Nilangekar wrote:
>> 2. The idea of having multi-threaded epoll based network client was to drive 
>> more throughput by using multiplexed epoll implementation and (fairly) 
>> distributing IOs from several vdisks (typical VM assumed to have atleast 2) 
>> across 8 connections. 
>> Each connection is serviced by single epoll and does not share its context 
>> with other connections/epoll. All memory pools/queues are in the context of 
>> a connection/epoll.
>> The qemu thread enqueues IO request in one of the 8 epoll queues using a 
>> round-robin. Responses are also handled in the context of an epoll loop and 
>> do not share context with other epolls. Any synchronization code that you 
>> see today in the driver callback is code that handles the split IOs which we 
>> plan to address by a) implementing readv in libqnio and b) removing the 4MB 
>> limit on write IO size.
>> The number of client epoll threads (8) is a #define in qnio and can easily 
>> be changed. However our tests indicate that we are able to drive a good 
>> number of IOs using 8 threads/epolls.
>> I am sure there are ways to simplify the library implementation, but for now 
>> the performance of the epoll threads is more than satisfactory.
> 
> By the way, when you benchmark with 8 epoll threads, are there any other
> guests with vxhs running on the machine?
> 

Yes. Infact the total througput with around 4-5 VMs scales well to saturate 
around 90% of available storage throughput of a typical pcie ssd device.

> In a real-life situation where multiple VMs are running on a single host
> it may turn out that giving each VM 8 epoll threads doesn't help at all
> because the host CPUs are busy with other tasks.

The exact number of epolls required to achieve optimal throughput may be 
something that can be adjusted dynamically by the qnio library in subsequent 
revisions. 

But as I mentioned today we can change this by simply rebuilding qnio with a 
different value for the #define

Ketan
> Stefan