Pruning Circuits

On Tuesday, my former PhD student Erwei Wang (now at AMD) will present our recent paper “Logic Shrinkage: Learned FPGA Netlist Sparsity for Efficient Neural Network Inference” at the ACM International Symposium on FPGAs. This is joint work with our collaborator Mohamed Abdelfattah from Cornell Tech as well as James Davis, George-Ilias Stavrou and Peter Y.K. Cheung at Imperial College.

In 2019, I published a paper in Phil. Trans. Royal Soc. A, suggesting that it would be fruitful to explore the possibilities opened up when considering the graph of a Boolean circuit – known as a netlist – as a neural network topology. The same year, in a paper at FCCM 2019 (and then with a follow-up article in IEEE Transactions on Computers), Erwei, James, Peter and I showed how to put some of these ideas into practice by learning the content of the Boolean lookup tables that form the reconfigurable fabric of an FPGA, for a fixed circuit topology.

Our new paper takes these ideas significantly further and actually learns the topology itself, by pruning circuit elements. Those working on deep neural networks will be very familiar with the idea of pruning – removing certain components of a network to achieve a more efficient implementation. Our new paper shows how to apply these ideas to prune circuits made of lookup tables, leaving a simplified circuit capable of highly-efficient inference. Such pruning can consist of reducing the number of inputs of each Boolean LUT and, in the limit, removing the LUT completely from the netlist. We show that very significant savings are possible compared to binarising a neural network, pruning that network, and only then mapping it into reconfigurable logic – the standard approach to date.

We have open-sourced our flow, and our paper has been given a number of ACM reproducibility badges. Please try it out at https://github.com/awai54st/Logic-Shrinkage and let us know what you think. And, if you’re attending FPGA next week, reach out and say hello.

Better accuracy / silicon area tradeoffs through pruning circuits

Islands of Certainty

Readers of this blog may remember that my PhD student Jianyi Cheng (jointly supervised by John Wickerson) has been working on high-level synthesis, combining dynamic scheduling with static scheduling. His latest contribution, to be presented on 28th February at the ACM FPGA conference, is about finding islands of static control flow in a sea of dynamic behaviour.

Here’s the story so far:

In 2018, our collaborators Lana Josipović and Paolo Ienne published some interesting work on dynamically-scheduled HLS which piqued my interest at the time.
In 2020, we worked with them to show that it’s possible to combine dynamic and static scheduling in HLS to achieve performance near that of the dynamic schedule and area near that of the static schedule. While we automated the production of such a combined circuit, what this work left unsolved was the question of how to identify the parts that would work best with static scheduling.

So now, two years later, we are back at the same conference to present a method to do just that. We now have an automated flow to select parts of a program to statically schedule, resulting in a 4x reduction in area combined with a 13% boost in performance compared to a fully dynamic circuit, a result that is close to the best achievable — as shown by exhaustively enumerating different parts of the program to schedule statically.

The basic idea of the paper is to develop the concept of a static island — a part of a dataflow graph where making decisions on scheduling of operations once, at compile time, is likely to have minimal impact on performance (or may even improve it) while opening the door to static resource sharing. We can throw a boundary around these islands, synthesise them efficiently with commercial HLS tools (we use Xilinx Vitis HLS), and integrate the result into the overall dynamic circuit using our previous open-source compiler flow.

So what makes a good static island? Unsurprisingly, these islands should exhibit static control flow or control flow with balanced path timing, e.g. in a conditional statement the if and else branch should take the same time, and loops should have constant dependence distances (or none at all). Jianyi also shows that there is an advantage to having these islands consume their inputs at offset-times, e.g. for a two-input island we may wish the static scheduler to be aware that second input is available — on average — two cycles after the first. He shows precisely how to generate ‘wrapper’ circuits for these components, allowing them to communicate with a dynamically scheduled environment.

The overall design flow, shown below, is now fully automated – freeing the user from writing the pragmas we required two years ago.

MATs, SATs and Maintained Schools

This post forms the second in a short series of blog posts about the current governance structure of schools in England, ahead of the Government’s expected while paper this spring. The previous post was about the review of the Education and Skills Funding Agency. In this post I focus on the landscape of school structures: multi-academy trusts (MATs), single-academy trusts (SATs) and local authority (LA) maintained schools from the perspective of a governor / trustee.

I will draw heavily on Sam Freedman’s report this month released by the Institute for Government as well as the excellent discussion in the Confederation of School Trusts’ report “What is a Strong Trust?“. One does not have to completely agree with Freedman’s or the CST’s perspectives to find both reports clear, well-argued, and a very useful step towards a greater debate in the education sector.

Freedman notes what is obvious to anyone who has worked in or with the sector – that the current dual system of maintained schools and academies leads to significant duplication and inefficiency. He notes that the regulatory system for academies is ‘incoherent’, largely as a result of the ESFA/DfE split I dealt with in my last blog post. He mentions – quite rightly – that LAs’ statutory responsibility for SEND and certain safeguarding requirements further complicates the picture when they have no effective oversight or intervention powers over academy trusts.

Early Academisation Errors

My experience is that the rapid expansion of the academies programme after the Academies Act 2010 was mismanaged. We have been left with a patchwork of schools and a web of contractual agreements. Schools which converted early have often been left with legacy articles of association based on the early model articles which demonstrated little insight into how schools could evolve under poor governance (modern model articles are much improved, though not perfect). Regulators have been left with very limited powers to improve matters, and it should not have come as a surprise to government that – as Freedman states – “[By 2012/3] with things moving so fast it quickly became apparent that the Department for Education (DfE) was becoming overwhelmed and could not properly oversee that many schools.” The unfortunate reality is that many schools are still stuck with the legacy of poor decisions made during this period.

Three School Structures

There are currently three main models for schools in England: those which are part of a multi-academy trust (MAT), those which are a single-academy trust (SAT) and those maintained by local authorities. Often in these discussions the first two types of school are lumped together, and it becomes a discussion about academies versus non-academies, but I think these three situations deserve distinct analysis. In particular, oversight of an individual school’s performance in the maintained sector and in the MAT sector is, in my experience, stronger than in the SAT sector, which is the outlier in terms of sufficient oversight. I wonder how many SATs are maintaining an Ofsted grade above inadequate, and therefore not subject to intervention, but are nevertheless not performing at the level they might be if they had closer interaction with other schools.

It is clear to anyone who has worked with or for the education sector that schools benefit immensely from working together and supporting each other, and I agree with Leora Cruddas’s argument made at a governance meeting I attended last year that, in times of difficulty for a school, a compulsion to work with other schools is important. At the moment, this primarily comes through membership of a strong multi-academy trust, though I do not see why a strong, well-resourced and empowered Local Authority could not form an equally viable mechanism to drive school improvement.

Pragmatics: Control and Intervention

Unsurprisingly, Freedman’s paper seeks to advise Zahawi on an appropriate way forward to a more coherent fully academised education system, and without entering the discussion over whether academy trusts are the best structure to deliver school education in the first place, it is worth engaging with the recommendations he makes.

Freedman sees the future of LAs – as per the 2016 white paper – as ensuring that every child has a school place, ensuring the needs of vulnerable children are met, and acting as champions for all parents and families, and – quite reasonably – proposes greater powers for LAs to ensure they can actually fulfil these objectives.

There are some proposals made in the paper that I would fully support:

Setting a transparent framework of expectations for MATs and giving the regulator powers to intervene, close or merge MATs for either financial/compliance reasons or educational reasons, not only tied to Ofsted grades.
Ensuring that MATs take ownership of educational improvement and are not simply back-office consolidation bodies as is sometimes the case currently.
Giving local authorities the right of access to MAT data.
A single regulator for academies, ideally organised as an arm’s length body.

There are also some proposals that are less clear cut for me:

Giving LAs the power to direct a change in individual academy PANs and admissions policies. Let’s assume that we move to an “all-MAT” system with LA’s still retaining the statutory duty to ensure a place for every pupil. To ensure clear lines of accountability, it seems appropriate for these to be at MAT level not individual academy level: surely mechanisms can still be put in place for intervention at MAT level to ensure they play their part in the strategic place-planning of LAs, rather than micromanaging a MAT’s academies over the head of its trustee board?
Moving SEND and exclusions policy monitoring / appeals from ESFA to LAs. I agree that ESFA is an odd place for this to sit at the moment in terms of ensuring joined up working between the RSC offices, ESFA and the Local Authority. But moving this to LAs rather than to the DfE again seems to introduce dual lines of accountability for MATs; might it not be better for RSCs to be required to ensure MATs meet the LA’s planning needs?
Giving individual academies a mechanism to argue to ‘break free’ from a MAT, involving giving schools legal status independent from the MAT. I agree that there may be very good reasons for academies to want to move to another MAT if the MAT is not functioning well, however under an all-MAT system it seems that a more appropriate approach is to provide the regulator with powers to intervene at MAT level than to provide independent legal status to individual academies.

There is an important question of democratic control, which I believe is required to balance some of these suggestions. In particular: who gets to appoint trustees (and members) of an academy trust, and what geographical area is it reasonable for a MAT to cover? On the first point, in the early days of academisation, academies needed to have some staff trustees, some (elected) parent trustees and a trustee appointed by the Local Authority. The Secretary of State was empowered to change the composition under specific conditions laid out in the academy’s funding agreement / articles of association. Government views on this composition has changed over time, with parent trustees going out of and then back into fashion, while staff trustees are definitely out of fashion at the moment. Local Authorities no longer get to appoint trustees at all in recent model articles. The situation locally will vary from trust to trust, depending on when their articles were approved — differences that cannot be justified, in my view. I would suggest that trusts articles are updated and that the ability (though not the requirement) for local authorities and the DfE (via RSC offices) to appoint trustees is included in the new model. This would provide LAs and the DfE direct information on trusts, rather than having to rely on existing trust boards to provide accurate information, in addition to providing a powerful mechanism for spreading best practice across trusts.

There is a huge opportunity for development of the schools sector in England. I look forward to publication of the white paper!

Appendix: Minor Quibbles

It’s probably worth pointing out a couple of very minor inaccuracies in the Institute for Government report:

Financial Notices to Improve, mentioned in the report, no longer exist since September 2021, precisely in recognition of the broader ESFA role currently; they are now subsumed within the broader “Notice to Improve” title.
A few times in the report, the ‘Special Measures’ category is cited as giving the Regional Schools Commissioners power to re-broker academies. While there may be additional powers – depend on the trust’s funding agreement – under a Special Measures Ofsted category, it’s clear in the Schools Causing Concern guidance that RSCs have the power to re-broker any inadequate school, i.e. also those judged to have ‘Serious Weaknesses’.

Restructuring Education

In the run-up to the Government’s planned white paper on education, I hope to be publishing a few brief blog posts on the landscape of education leadership and management in England. This post focuses on the summary findings from the ESFA review led by Sir David Bell and published this week.

Summary: this review is very welcome and I am supportive of all the key recommendations.

My perspective is one of governance: I have been a long-term member of Essex Schools Forum, first as an elected representative of maintained primary schools, then as an elected representative of secondary academies. Throughout this time (and beyond) I have been involved in helping Essex shape its local funding formula and respond to the national funding formula. I have governance experience both in the maintained sector and in a single academy trust, and work with a new post-16 multi-academy trust through my workplace.

From the perspective of accountability of academy trusts, I think such a review has long been overdue, in particular over clarity of roles between the offices of the regional schools commissioners and the lines of accountability through academy funding agreements.

The findings suggest retaining ESFA’s funding delivery function as an Arms Length Body (ALB) while moving a considerable number of responsibilities to the DfE. This seems sensible. In particular, from a governance perspective, I wholeheartedly endorse the finding that “The Regional School Commissioners (RSCs) and ESFA work together to provide oversight of the school system; the RSCs focus on educational performance, ESFA on financial management, with both contributing to governance. This sometimes creates points of friction internally and a lack of clarity externally”. The proposal, to move all governance oversight not required to be at ESFA to the new regional DfE structures, also seems entirely reasonable. The review also recommends clarifying the relationship between Ofsted, the DfE and ESFA – my experience of this is that there is already clarity over the different roles of Ofsted versus the DfE and ESFA, although admittedly this knowledge is not widespread, even amongst school leaders.

In line with this move to the DfE, the proposal to move ownership of the Academy Trust Handbook to the DfE (unless scaled back to a purely financial management document) is also to be welcomed by governors and trustees.

The final sensible proposal I would highlight from the review aims to achieve greater alignment in dealing with complaints between the maintained and academy sector. As part of this process, I would urge the DfE to consider mandating a more uniform complaints policy at trust level for academies: although the model DfE policies are entirely reasonable, they are not statutory, and academies minimally complying with legislation set out in the Education (Independent School Standards)(England) Regulations 2014 essentially force complaints to be dropped or escalated to Ofsted or ESFA which could be dealt with at trust level under better procedures.

Of course there are bigger questions relating to the role of multi-academy trusts and local authorities and their interaction with the department for education, and I hope to cover some of these issues in future blog posts. But within the confines of our current system, these reforms seem very much worthwhile.