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=== NVMe Emulation Performance Optimization ===
=== NVMe Emulation Performance Optimization ===
'''Summary:'''
'''Summary:'''
QEMU's NVMe emulation uses the traditional trap-and-emulation method to emulate I/Os, thus the performance suffers due to frequent VM-exits. It is possible to run emulation in a dedicated thread called an IOThread using the "ioeventfd" mechanism to receive notifications of Submission Queue Tail Doorbell writes.  Emulating NVMe in a separate thread allows the vcpu thread to continue execution and results in better performance.
QEMU's NVMe emulation uses the traditional trap-and-emulate method to
emulate I/Os, thus the performance suffers due to frequent VM-exits.
Version 1.3 of the NVMe specification defines a new feature to update
doorbell registers using a Shadow Doorbell Buffer. This can be utilized
to enhance performance of emulated controllers by reducing the number of
Submission Queue Tail Doorbell writes.


The NVMe Specification 1.3 defines a new feature to update doorbell registers using a Shadow Doorbell Buffer. This can be utilized to enhance performance of emulated controllers by reducing the number of Submission Queue Tail Doorbell writes.
Further more, it is possible to run emulation in a dedicated thread
called an IOThread. Emulating NVMe in a separate thread allows the vcpu
thread to continue execution and results in better performance.


Finally, it is possible for the emulation code to watch for changes to the queue memory instead of waiting for doorbell writes. This technique is called polling and reduces notification latency at the expense of an another thread consuming CPU to detect queue activity.
Finally, it is possible for the emulation code to watch for changes to
the queue memory instead of waiting for doorbell writes. This technique
is called polling and reduces notification latency at the expense of an
another thread consuming CPU to detect queue activity.


The goal of this project is to add implement these optimizations so QEMU's NVMe emulation performance becomes comparable to virtio-blk performance.
The goal of this project is to add implement these optimizations so
QEMU's NVMe emulation performance becomes comparable to virtio-blk
performance.


Tasks include:
Tasks include:
* Add Submission Queue Tail Doorbell register ioeventfd support (see existing patch linked below)
* Add Shadow Doorbell Buffer support to reduce doorbell writes
* Add Shadow Doorbell Buffer support to reduce doorbell writes
* Add Submission Queue Tail Doorbell register ioeventfd support when the Shadow Doorbell Buffer is enabled (see existing patch linked below)
* Add Submission Queue polling
* Add Submission Queue polling
* Add IOThread support so emulation can run in a dedicated thread
* Add IOThread support so emulation can run in a dedicated thread
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* https://vmsplice.net/~stefan/stefanha-kvm-forum-2017.pdf
* https://vmsplice.net/~stefan/stefanha-kvm-forum-2017.pdf
* https://patchwork.kernel.org/patch/10259575/
* https://patchwork.kernel.org/patch/10259575/
* https://lore.kernel.org/qemu-devel/1447825624-17011-1-git-send-email-mlin@kernel.org/T/#u


'''Details:'''
'''Details:'''
* Skill level: intermediate-advanced
* Skill level: intermediate-advanced
* Language: C
* Language: C
* Mentor: Paolo Bonzini <pbonzini@redhat.com> ("bonzini" on IRC), Stefan Hajnoczi <stefanha@redhat.com> ("stefanha" on IRC)
* Mentor: Klaus Jensen <its@irrelevant.dk> (kjensen on IRC)
* Suggested by: Huaicheng Li <huaicheng@cs.uchicago.edu>, Paolo Bonzini
* Suggested by: Huaicheng Li <huaicheng@cs.uchicago.edu>, Paolo Bonzini <pbonzini@redhat.com> ("bonzini" on IRC)

Revision as of 09:46, 22 February 2022

NVMe Emulation Performance Optimization

Summary: QEMU's NVMe emulation uses the traditional trap-and-emulate method to emulate I/Os, thus the performance suffers due to frequent VM-exits. Version 1.3 of the NVMe specification defines a new feature to update doorbell registers using a Shadow Doorbell Buffer. This can be utilized to enhance performance of emulated controllers by reducing the number of Submission Queue Tail Doorbell writes.

Further more, it is possible to run emulation in a dedicated thread called an IOThread. Emulating NVMe in a separate thread allows the vcpu thread to continue execution and results in better performance.

Finally, it is possible for the emulation code to watch for changes to the queue memory instead of waiting for doorbell writes. This technique is called polling and reduces notification latency at the expense of an another thread consuming CPU to detect queue activity.

The goal of this project is to add implement these optimizations so QEMU's NVMe emulation performance becomes comparable to virtio-blk performance.

Tasks include:

  • Add Shadow Doorbell Buffer support to reduce doorbell writes
  • Add Submission Queue Tail Doorbell register ioeventfd support when the Shadow Doorbell Buffer is enabled (see existing patch linked below)
  • Add Submission Queue polling
  • Add IOThread support so emulation can run in a dedicated thread

Links:

Details:

  • Skill level: intermediate-advanced
  • Language: C
  • Mentor: Klaus Jensen <its@irrelevant.dk> (kjensen on IRC)
  • Suggested by: Huaicheng Li <huaicheng@cs.uchicago.edu>, Paolo Bonzini <pbonzini@redhat.com> ("bonzini" on IRC)
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