Design of OpenMP-based PageRank algorithm for link analysis.
Unordered PageRank is the standard approach of PageRank computation (as described in the original paper by Larry Page et al. (1)), where two different rank vectors are maintained; one representing the current ranks of vertices, and the other representing the previous ranks. On the other hand, ordered PageRank uses a single rank vector, representing the current ranks of vertices (2). This is similar to barrierless non-blocking implementations of the PageRank algorithm by Hemalatha Eedi et al. (3). As ranks are updated in the same vector (with each iteration), the order in which vertices are processed affects the final result (hence the adjective ordered). However, as PageRank is an iteratively converging algorithm, results obtained with either approach are mostly the same.
In this experiment (approach-ordered), we compare the performance of
ordered and unordered OpenMP-based PageRank (and compare it alongside
ordered and unordered sequential PageRank). A schedule of dynamic, 2048
is used for OpenMP-based PageRank as obtained in (4). We
use the follwing PageRank parameters: damping factor α = 0.85, tolerance
τ = 10^-6, and limit the maximum number of iterations to L = 500. The error
between the current and the previous iteration is obtained with L1-norm, and
is used to detect convergence. Dead ends in the graph are handled by always
teleporting any vertex in the graph at random (teleport approach (5)).
Error in ranks obtained for each approach is measured relative to the unordered
sequential approach using L1-norm.
From the results, we observe that the ordered OpenMP-based approach is somewhat faster than the unordered approach in terms of time, and follows a trend similar to that of sequential PageRank. However, the ordered approach (both OpenMP-based and sequential) converges in significantly fewer iterations than the unordered approach. This indicates that the ordered approach could have been quite a bit faster, but is not, because of some overhead (possibly cache coherence overhead due to parallel read-write access to the same vector). In any case, ordered PageRank is indeed faster than unordered Pagerank.
In this experiment (adjust-tolerance-ordered), we perform OpenMP-based
ordered PageRank while adjusting the tolerance τ from 10^-1 to 10^-14
with three different tolerance functions: L1-norm, L2-norm, and L∞-norm.
We also compare it with unordered PageRank (both OpenMP-based and sequential)
for the same tolerance and tolerance function. We use a damping factor of
α = 0.85 and limit the maximum number of iterations to L = 500. The error between
the approaches is calculated with L1-norm. The sequential unordered approach
is considered to be the gold standard (wrt to which error is measured). Dead ends
in the graph are handled by always teleporting any vertex in the graph at
random (teleport approach [(4)]). The teleport contribution to all vertices is
calculated once (for all vertices) at the begining of each iteration.
From the results, we observe that OpenMP-based ordered PageRank only
converges faster than the unordered approach below a tolerance of
τ = 10^-6. This may be due to cache coherence overhead associated with the
ordered approach, which can exceed the benefit provided by ordered approach with
loose tolerance values. In terms of the number of iterations, we interestingly
observe that iterations of OpenMP-based unordered/ordered approaches are higher
than with sequential approaches. We currently do not have an explanation for
this.
In this experiment (adjust-schedule), we compare performance obtained for
OpenMP-based PageRank for various schedules. Each thread is assigned a
certain number of vertices to process. The schedule kind is adjusted among
static / dynamic / guided / auto, and the chunk size is adjusted
from 1 to 65536. We do this for the rank computation step. PageRank
factors, contributions, and teleport contribution computation is calculated with
suitable OpenMP schedule (auto). We use the follwing PageRank parameters:
damping factor α = 0.85, tolerance τ = 10^-6, and limit the maximum number
of iterations to L = 500. The error between the current and the previous
iteration is obtained with L1-norm, and is used to detect convergence.
From the results, we observe that a dynamic schedule with a chunk size of
2048 appears to perform the best. This however may change based on the
size of graphs in the dataset, or the system used. In such cases auto schedule
may be used as a fallback. We also observe that the difference in ranks
obtained from sequential and OpenMP-based approach is relatively high
(< 10^-3) on large directed graphs. This may be due to the fact that parallel
reduce performed for teleport contibution calculation differs from sequential
reduce due to inaccuracies associated with 32-bit floating point format
(float), and can be avoided by using 64-bit floating point format (double).
This experiment (approach-hybrid) was for comparing the performance between
finding pagerank using uniform OpenMP (all routines use OpenMP), or using
hybrid OpenMP (some routines are sequential). Both techniques were
attempted on different types of graphs, running each technique 5 times per graph
to get a good time measure. Number of threads for this experiment (using
OMP_NUM_THREADS) was varied from 2 to 48.
It appears that hybrid approach performs worse in most cases, and only slightly better than uniform approach in a few cases. I am not sure why that is the case, possibly there could be some correlation between execution time and some other parameter. Note that neither approach makes use of SIMD instructions which are available on all modern hardware.
This experiment (compare-sequential) was for comparing the performance between
finding pagerank using a single thread (sequential), or finding pagerank
accelerated using OpenMP. Both techniques were attempted on different types
of graphs, running each technique 5 times per graph to get a good time measure.
Number of threads for this experiment (using OMP_NUM_THREADS) was varied from
2 to 48.
OpenMP does seem to provide a clear benefit for most graphs (except for the smallest ones). This speedup is definitely not directly proportional to the number of threads, as one would normally expect (Amdahl's law). Note that there is still room for improvement with OpenMP by using sequential versions of certain routines instead of OpenMP versions because not all calculations benefit from multiple threads (ex. vector-multiplication-openmp). Also note that neither approach makes use of SIMD instructions which are available on all modern hardware.
- An Efficient Practical Non-Blocking PageRank Algorithm for Large Scale Graphs; Hemalatha Eedi et al. (2021)
- PageRank Algorithm, Mining massive Datasets (CS246), Stanford University
- The PageRank Citation Ranking: Bringing Order to the Web; Larry Page et al. (1998)
- The University of Florida Sparse Matrix Collection; Timothy A. Davis et al. (2011)
- What's the difference between "static" and "dynamic" schedule in OpenMP?
- OpenMP Dynamic vs Guided Scheduling
